Amino Acid Metabolism

xiaoli

Protein metabolism

Reviews:

C

R

H

NH3

COOmetabolism

synthesis

catabolism

Digestion and absorption of protein

Normal metabolism of amino acids

Special products of amino acids

Major content

Proteins play a major role in ensuring your health well being. There are innumerable functions of proteins in the body.

protein makes up nearly 17 percent of the total body weight. For example: muscle contains about 1/3 protein, bone about 1/5 part and skin consists of 1/10 portion. The rest part of proteins is in the other body tissues and fluids.

building and repairing of body tissues.

Take part in some kinds of important physiological activities

Nutritional Function of Protein

Oxidation and supply energy

regulation of body processes and formation of enzymes and hormones, antibody. There are distinctive kinds of proteins, each performing a unique function in the body.

1. Nitrogen balance

the balance between the amount of nitrogen ta

ken in (foods or the body) and the amount given of

f (lost or excreted)

Significance:

Measuring the amount of intake and losses of total nitrogen can help us to know the general situation of protein metabolism.

How to assess the condition of protein metabolism?

★ positive: synthesis > degradation

(e.g., growth, body building)

★ negative: synthesis < degradation

(e.g., starvation, trauma, cancer cachexia)

★ Equilibrium: synthesis = degradation

(healthy adults eating a balanced diet)

nitrogen balance

2. Physical requirements of proteins Lowest requirement:

30~50g/day Recommend requirement:

80g/day (65kg man)

Amino acids are not

stored by the body,

must be obtained from

the diet, synthesized

de novo.

Some sources of dietary protein include:

Meat, poultry and fish

Eggs, Dairy products

Seeds and nuts

Beans and lentils

Soy products

Grains, especially wheat and rice,

barley and corn.

3. Nutrition value of proteins

Eight amino acids are generally regarded as essential for humans:

phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and lysine

(1) Essential amino acids :

some Amino acids that cannot be synthesized by the body and must be obtained from the diet.

(2) Non- essential amino acids

other 12 kinds of AAs, the non-essential or dispensable amino acids can be synthesized in the body either other roadways.

Note: a Arg is synthesized in the urea cycle, but the rate is too slow

to meet the needs of growth in children

b Met is required to produce cysteine if the latter is not supplied adequately by the diet.

c Phe is needed in larger amounts to form tyr if the latter is not supplied by the diet.

His and Arg are essential AAs for infants and children.

(4) nutrition value A protein’s nutritional value is judged by how

many of the essential amino acids it provides and

in what quantity.

Different foods contain different numbers and a

mounts of the essential amino acids.

lysine tryptophan

(5) Complementary effect of dietary proteins

Two or more plant proteins are consumed together which complement each other in essential amino acid content.

Digestion Absorption Putrefaction of protein

hydrolysisAmino acids absorb

Dietary proteinDietary protein

Significance:

◆ Large small Help to absorb

◆ eliminate the species specificity and antigenic

ity, avoid allergy , toxic reaction.

2.1 Digestion

Pepsin

Chymotrypsin, trypsin,and exopeptidases

Amino acids

site: stomach, small intestine

Proteolytic enzymes of pancreatic juice

HCl from parietal cellsStomach pH 1.6 to 3.2Pepsinogen from chief cells

Pepsinogen

HClPepsin

enzymes: pepsin

Initiated in stomach

The substrate mainly are phenylala

nine,tyrosine,tryptophan

Aromatic amino acids Products:

insoluble protein, soluble protein,

polypeptides and amino acids

Protein Digestion – Small Intestine

Pancreatic enzymes secreted

trypsin ChymotrypsinCarboxypeptidaseelastase

Zymogens

Trypsinogen ChymotrypsinogenprocarboxypeptidaseProelastase

Zymogens

A zymogen is the inactive precursor of an enzyme.

Activation of zymogen

A inactive zymogen become active enzyme.

In a zymogen, a peptide blocks the active site of the enzy

me. Cleaving off this peptide activates the enzyme.

1. avoids self-digestion:

This is necessary to prevent the digestive enzymes

from autodigesting the cells that produce them.

2. stored and transported safely : The body typically secretes zymogens rather than active enzymes because they can be stored and transported safely without harm to surrounding tissues, and released when conditions are favorable for optimal activity.

Significance:

This cleavage renders the zymogen a functional enzyme by changing the shape of the peptide and forming the active site where enzymatic action will occur.

The molecule is composed of amino acids strung together into a peptide. When the zymogen is in the presence of protease, some of the amino acids are removed.

In a zymogen, a peptide blocks the active site of the enzyme. Cleaving off this peptide activates the enzyme.

protease

active site

trypsintrypsinogen

enterokinase

chymotrypsinogen

elastase

procarboxypeptidase

chymotrypsin

proelastase

carboxypeptidase

cascade reaction

Amplification effect

Proteolytic enzymes of pancreatic juice

endopeptidases

exopeptidases

trypsin: Arg, Lys (C)

chymotrypsin: Tyr, Trp, Phe, Met, Leu (C)

elastase: Ala, Gly, Ser (C)

carboxypeptidase

aminopeptidase

Protein Digestion – Small Intestine

H2N-CH-C-NH-CH---

R2R1 RnR Rn-1

O O O

amino peptidase endopeptidase carboxy peptidase

amino acid + H2N-CH-C-NH-CH-COOH

R R

O

dipeptidase

amino acid

polypeptide

dipeptide

NH-CH-C-NH-C--- NH-CH-C-NH-CH-COOH

Protein Digestion

Proteins are broken down to

Tripeptides

Dipeptides

Free amino acids

2.2 absorption

Free amino acids Absorption

★ Carrier systems

★ Meister cycle/ γ-glutamyl cycle transport amino acids

Amino acids

Na+

Amino acids

Na+

carrier protein

Lumen(small intestine)

Brush broad membrance

Na+ pump

ATP

Free Amino Acid Free Amino Acid AbsorptionAbsorption

Na+

Amino acids

Carrier systems Neutral AA Basic AA Acidic AA Amino acids

Entrance of some AA is via active transport

Requires energy

Meister cycle/ γ-glutamyl cycle tran

sport amino acids

γ-glutamyl cycle include two steps ：

• GSH(glutathione) transport amino acids• GSH synthesis

cysteineglycine

peptase 5-pidolic acid

AA

H2NCH

COOH

R

glutamic

γ-glutamylcyclotransferase

5-oxoproline ATP

ADP+Pi

γ-glutamylcysteine

γ-glutamylcysteine synthetase(γ-GCS)

ADP+Pi

ATPglutathione synthetase

ATPADP+Pi

extracellular

γ-glutamyl transferase

Cell membrance

intracellular

γ-glutamyl cycle / Meister cycle

γ-glutamyl amino acid

Cys-Gly

GSH

COOH

CHNH2

CH2

CH2

C

O

NH CH

COOH

RCHH2N

COOH

R

AA

目 录

Peptide Absorption Form in which the majority of

protein is absorbed More rapid than absorption of

free amino acids Active transport

Energy required Metabolized into free amino

acids in enterocyte Only free amino acids

absorbed into blood

§2.3 Putrefaction of proteins

Some undigested proteins and no absorbed

products are anaerobic decomposed by the bact

eria in intestine.

The products are toxic to body except

few vitamin and fatty acid.

Putrefaction of proteins:

1. Production of amines

R

NH2

CO2

R

amino acid

bacteria

amine

CH COOH CH2 NH2

histidine histamine

tryptophan

tryptamine

tyrosine tyromine

β-hydroxytyramine

CO2+H2Oliver

phenylalanine phenylethylamine phenolethanolamine

CO2+H2Oliver

noradrenalin dopamine

tyramine

CH2

CH2NH2

OH

CH2

CH2NH2

OH β-hydroxytyramine

CH2NH2

C OHH

OH

CH2NH2

C OHH

OH

phenolethanolamine

CH2NH2

C OHH

CH2NH2

C OHH

phenylethylamine

CH2

CH2NH2

CH2

CH2NH2

hydroxylase hydroxylase

noradrenalin

dopamine

•false neurotransmitter

is a chemical compound which closely imitates the action of a neurotransmitter in the nervous system,but that has no or little effect on postsynaptic receptors.

β-hydroxytyramine

CH2NH2

C OHH

OH

CH2NH2

C OHH

OH

phenolethanolamine

CH2NH2

C OHH

CH2NH2

C OHH

3. Some other toxic materials Tyr → phenol Trp → indole Cys → hydrogen sulfide (H2S)

Two sources:

(1) Metabolism on unabsorbed amino acids

(2) Urea hydrolyzed by urease

2. Production of ammonia (NH3)

General Metabolism of

Amino Acid

§ 3.1 Protein turnover

the balance between protein synthesis and protein degradation .

Rapid protein turnover ensures that some regulatory

proteins are degraded so that the cell can respond to

constantly changing conditions.

In healthy adults, the total amount of protein in the

body remains constant, because the rate of protein sy

nthesis is just sufficient to replace the protein that is d

egraded. this process is called protein turnover.

half-life

Examples of protein turnover in the body

Half-life is the period of time it takes for a substance undergoing decay to decrease by half.

§ 3.2 Degradation of protein in cells

1. Lysosomal pathway Extracellular proteins, membrane-associat

ed proteins and long-lived proteins

ATP-independent process

Enzyme: Cathepsins

2. Cytosol pathway

Abnormal proteins, damaged proteins and short-lived proteins

ATP ubiquitin

Proteasome

enzyme

7~9 residues peptides

ubiquitination

ubiquitin-proteins

Ubiquitin (Ub) is a small protein that is composed of 76 a

mino acids; exists in all eukaryotic cells, only in eukaryotic

organisms.

Among eukaryotes, ubiquitin is highly conserved, meaning

that the amino acid sequence does not differ much when ver

y different organisms are compared. For example, there are

only 3 differences in the sequence when Ub from yeast is co

mpared to human Ub.

ubiquitin ubiquitious

Ubiquitin performs its myriad functions through conjugation to a large range of target proteins.

ubiquitination

ubiquitin + E1

ATP AMP+Pi

ubiquit -E1Activate ubiquitin

ubiquitin -E1

E2 E1

ubiquitin -E2Ub-conjugating enzymes

1. E1 enzymes known as Ub-activating enzymes. These enzymes modify Ub so that it is in a reactive state (making it likely that the C-terminal glycine on Ub will react with the lysine side-chains on the substrate protein).

2. E2 enzymes known as Ub-conjugating enzymes. These enzymes actually catalyze the attachment of Ub to the substrate protein

3. E3 enzymes known as Ub-ligases. E3's usually function in concert with E2 enzymes, but they are thought to play a role in recognizing the subtrate protein.

ubiquitin -E2

E2

proE3 Ubiquitin-pro

proteasome Degratation(7~9 residues peptides)

Ubiquitin-pro

Ub-ligases

The general reaction pathway is shown in the figure below. First, Ub i

s activated by E1 in an ATP-dependent fashion.

E2 and E3 then work together to recognize the substrate protein and

conjugate Ub to it. Ub can be attached as a monomer or as a previousl

y synthesized chain (as shown).

From this point, the ubiquinated protein is shuttled to the proteasome

for degradation

Degradation of protein in cells

amino acids in intracellular and extracellular fluids.

Amino acid pool:

Amino acids%

muscle 50%

liver 10%

kidney 4%

blood 1~6%

Synthesis of proteins

Fates of amino acidsSources of amino acids

§ 3.1 The sources and fates of AAs

Amino acidmetabolic pool

deamination

decarboxylation

NH3

α-Keto acid

Ketone bodies

Oxidation

Glucose

Urea

AmineCO2

conversion

Non- protein nitrogen compounds

absorption

degradation

synthesis

Dietary proteins

Tissue proteins

Amino acids synthesized

§ 3.3 The catabolism of AAs

1. Deamination of AAs

Four types:

transamination

oxidative deamination

non-oxidative deamination

union deamination

(1) Transamination

Transamination is the process by which an amino group, usually from glutamate, is transferred to an α-keto acid, with formation of the corresponding amino acid plus α-ketoglutarate.

aminotransferase

Key points:

① reversible:Transaminases (aminotransferases) catalyze the reversible reaction at right.

② Lys and Pro cannot be transaminated.

③ Aminotransferases utilize a coenzyme - pyridoxal phosphate - which is derived from vitamin B6.

The prosthetic group of Transaminase is pyridoxal phosphate (PLP), a derivative of vitamin B6.

p y rid o x a l p h o sp h a te (P L P )

NH

CO

P

O O

O

O H

C H 3

CH O

H 2

pyridoxamine phosphate

α-keto acid

Schiff baseAmino acid pyridoxal phosphate

Isomer of Schiff base

The amino group remains on what is now pyridoxamine phosphate (PMP). A different -keto acid reacts with PMP and the process reverses, to complete the reaction.

NH

CO

P

O O

O

OH

CH3

CH2

NH2

H2

R C COO

O

Enz Lys NH2

Pyridoxamine phosphate (PM P)

-keto acid

What was an amino acid leaves as an -keto acid.

Transaminases equilibrate amino groups among available -keto acids.

This permits synthesis of non-essential amino acids, using amino groups from other amino acids & carbon skeletons synthesized in a cell.

Thus a balance of different amino acids is maintained, as proteins of varied amino acid contents are synthesized.

Although the amino N of one amino acid can be used to synthesize another amino acid, N must be obtained in the diet as amino acids (proteins).

In addition to equilibrating amino groups among available -keto acids, transaminases funnel amino groups from excess dietary amino acids to those amino acids (e.g., glutamate) that can be deaminated.

Carbon skeletons of deaminated amino acids can be catabolized for energy, or used to synthesize glucose or fatty acids for energy storage.

Only a few amino acids are deaminated directly.

1. GPT (serum glutamate pyruvate transaminase)

GPTGPT(ALT)(ALT)

/ Alanine transaminase/ Alanine transaminase ( (ALTALT))

Two important transaminases:

/ Aspartate aminotransferase (AST)

2. GOT (serum glutamate oxaloacetate transaminase)

GOT(AST)

organ GOT GPT

heart 156000 7100

liver 142000 44000

skeletal 99000 4800

kidney 91000 19000

organ GOT GPT

pancrease

spleen

lung

serum

28000 2000

14000 1200

10000 700

20 16

Elevated levels of ALT may indicate : alcoholic liver disease cancer of the liver cholestasis or congestion of the bile ducts cirrhosis or scarring of the liver with loss of function death of liver tissue Hepatitis or inflammation of the liver noncancerous tumor of the liver use of medicines or drugs toxic to the liver

！！ Therefore, when the liver is injured, ALT is released into the bloodstream.

ALT is an enzyme produced in hepatocytes and is highly c

oncentrated in the liver.

AST also reflects damage to the hepatic cells and

is less specific for liver disease. It can also be released

with heart, muscle and brain disorders.

Therefore, this test may be ordered to help diagnose

various heart, muscle or brain disorders, such as a myoc

ardial infarct (heart attack).

ALT: Alanine aminotransferase (in liver)

AST: Aspartate aminotransferase (in heart)

Two important transaminases:

pyruvate

alanine

glutamate

-ketoglutarate

oxaloacetate

aspartate

ALT AST

No net removal of N from the amino acid pool.

(2) Oxidative deamination

2. It is one of the few enzymes that can use NAD+ or NADP+ as e- acceptor.

Oxidation at the α-carbon is followed by hydrolysis, releasing NH4

+.

1. Glutamate Dehydrogenase catalyzes a major reaction that effects net removal of N from the amino acid pool.

Some other pathways for deamination of amino acids:

1. Serine Dehydratase catalyzes: serine pyruvate + NH4

+

2. Peroxisomal L- and D-amino acid oxidases catalyze: amino acid + FAD + H2O -keto acid + NH4

+ + FADH2

FADH2 + O2 FAD + H2O2

Catalase catalyzes: 2 H2O2 2 H2O + O2

H O C H 2

HC C O O

N H 3+

C C O O

OH 2 O N H 4+

C C O O

N H 3+

H 2 C H 3 C

H 2 O

s e r i n e a m i n o a c r y l a t e p y r u v a t e S e r i n e D e h y d r a t a s e

R-CH-COOH

NH2

R-C-COOHO

COOH

CH2

COOH

C O

2

COOH

CH2

COOH

CHNH2

2 NAD+ + H2O

NADH + H+ + NH3

¦Á-ketoglutarate¦Á-amino acid

¦Á-keto acid

L-glutamate dehydrogenasetransaminase

Glu

(3) Union deamination

The α- amino group of most amino acids is transferred to α- ketoglutarate to form an α- keto acid and glutamate by transaminase. Glutamate is then oxidatively deaminated to yield ammonia and α- ketoglutarate by glutamate dehydrogenase.

Alanine + α-ketoglutarate Pyruvate + glutamate

Glutamate + NAD+ + H2O α-ketoglutarate + NADH + NH4

+

Net Reaction:

Alanine + NAD+ + H2O pyruvate + NADH + NH4+

(4) Purine nucleotide cycle (in muscle)

amino acid

COOH

(CH2)2

CO

COOH

COOH

(CH2)2

COOH

keto-glutarate¦Á-

ketoacid

¦Á-

L-Glu

trans-aminase

CHNH2

CH2COOH

COCOOH

oxaloacetate

HN

N N

N

O

R-5'-P

N

N N

N

R-5'-P

HOOCCH2CHCOOH

NH2

HOOCCH2CHCOOH

NH

adenylosuccinate

CH2COOH

CHOHCOOHmalate

CHCOOH

CHCOOHfumarate

N

N N

N

NH2

R-5'-P

H2O

NH3

AMP

IMP AMP deaminaseAST

Asp

adenylosuccinate synthetase

adenylo-succinase

Amino acid

C

R

H

NH3

COO

NH3urine

Ketone bodies

glucose

oxidation

deaminationliver

Section 4

Metabolism of Ammonia

1. Sources:

⑴ Endogenous sources:

① Deamination of AAs--main source

② Catabolism of other nitrogen containing compounds.

RCH2NH2 RCOH + NH3amine oxidase

§ 4.1 Source and outlet of ammonia (NH3)

CONH2

(CH2)2

CHNH2

COOH

+ H2OGlutaminase

COOH

(CH2)2

CHNH2

COOH

+ NH3

Gln Glu

③ Kidney secretion (Gln)

⑵ Exogenous sources ： ① Putrefaction in the intestine.

② Degradation of urea present in fluids secreted into the GI tract

soapsuds enema

NH3 is easy to dispersion, NH4+ is not .

pH<7 H+ + NH3 NH4+ expel

Liver desfunction

Reduce the absorption of ammonia:

weakly acidic dialysate in colonic dialysis

acidifying diuretic

alkaline dialysate ,alkaline medician

urea

inhibition

2. Outlets:

(1) Formation of urea

(2) Formation of Gln

(3) Excrete in urine ( NH4+ )

(4) Synthesis of AA

§ 4. 2 Transportation of NH3

1. Alanine-glucose cycle

2. Transportation of ammonia by Gln

1. Alanine-glucose cycle

protein

amino acid

NH3

pyruvate

¦Á-keto glutarate

G

muscle

pyruvate

G

NAD+ + H2O

NADH + H+

blood liver

urea

Glu

Ala Ala Ala

Glu

G

¦Á-keto glutarate

+ NH3

2. Transportation of ammonia by Gln

CONH2

(CH2)2

CHNH2

COOH

Gln synthetaseCOOH

(CH2)2

CHNH2

COOH

+ NH3

ATP ADP + Pi

Glu GlnGlutaminase

H2O

Urea is less toxic than ammonia.

The Urea Cycle occurs mainly in liver. ( ornithine cycle / Krebs cycle )

Most animals convert excess nitrogen to urea, prior to excreting it.

H 2 N C

O

N H 2

u r e a

§ 4. 3 Formation of urea

Transportation of NH3

liver

1. Site: liver (mitochondria and cytosol)

2. Process --------- Urea Cycle

CO2 + 2NH3 C=O+H2O

NH2

NH2

ornithine NH3 + CO2

H2O

NH3H2O

H2O

urea

arginase

Arg citrulline

① Formation of carbamoyl phosphate

(in mitochondria)

H2N-C-O~PO3H2

O2ATP 2ADP+Pi

NH3 + CO2 + H2O

carbamoyl phosphate

CPS I

Carbamoyl phosphate synthase backbone structure

• Tunnel connecting active sites (blue wire)

Carbamoyl phosphate synthase Ⅰ

Carbamoyl phosphate synthetase (CPS ) is an allosteⅠ Ⅰric enzyme and is absolutely dependent up on N-acetylgluta

mic acid (AGA) for its activity.

Carbamoyl phosphate synthetase :Ⅰ

Occurs in mitochondria of liver cells. It is involved in urea synthesis.

Carbamoyl phosphate synthetase :Ⅱ

Present in cytosol of liver cells which is involved in pyrimidine synthesis.

② Formation of citrulline

(in mitochondria)

H2N-C-O~PO3H2

O

carbamoyl phosphate

+

NH2

£¨CH £©2 3

CHNH2

COOHornithine

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C OPi

citrulline

OCT

OCT: ornithine carbamoyl transferase

③ Formation of arginine (in cytosol)

two sub-steps

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C O

citrulline

+

COOH

H2-N-C-H

CH2

COOH

ATP AMP+PPi

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C

COOH

N-C-H

CH2

COOH

arginino succinate

Asp

ASS

ASS: argininosuccinate synthetase

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C

COOH

N-C-H

CH2

COOH

arginino succinate

ASL

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C

COOH

CH

HC

COOH

NH

+

fumarate

Arg

ASL: argininosuccinate lyase

④ Formation of urea (in cytosol)

NH2

£¨CH £©2 3

CHNH2

COOHornithine

NH

£¨CH £©2 3

CHNH2

COOH

NH2

C NHH2O

arginase+

NH2

NH2

C O

urea

Arg

2ADP+Pi

CO2 + NH3 + H2O

Carbamoyl phosphate

2ATP N-acetylglutamic acid

Pi

ornithine citrulline

Amino acids

α-ketoglutaric acid

Glutamic Acid

α-keto acid

citrulline

Arginino succinate

AspATP

AMP + PPi

Arg

ornithine

urea

mitochondria

in cytosol目 录

fumarate

malic acid

oxaloacetic acid

Urea cycle

Summary of urea synthesis

One nitrogen of urea molecule comes from ammonia, another nitrogen comes from Asp.

HCO3- ion provides the carbon atom of urea.

Found primarily in liver and lesser extent in kidney

Synthesis of a urea will consume 3ATP and 4 ~P.

Total formula ：

H 2 N C

O

N H 2

u r e a

CO2 + NH3 + H2O

Regulation factors:

1. Ratio of protein in dietary foods:

2. Carbamoyl phosphate synthetase is allosterically a

ctivated by N-acetylglutamate

(acetyl CoA + glutamate N-acetylglutamate)

3. Rate limiting enzyme: argininosuccinate synthetase(ASS)

Clinical significance of urea

A moderately active man consuming about

300gm carbohydrates ,100gm of fats and 100gm

of proteins daily must excrete about 16.5gm of N

daily. 95% is eliminated by the kidneys and the remaini

ng 5%, for the most part as N, in the faeces.

in man ,normal blood level of NH3 varies from

40 to 70µg/100ml.free NH+4 concentration of fresh

plasma is less than 20µg per 100ml.

Normal blood ammonia level:

HYPERAMMONEMIAS

Ammonia has a direct neurotoxic effect on the CNS .for example ,elevated concentrations of ammonia in the blood cause the symptoms of ammonia intoxication, which include:

tremors,slurring of speech,Somnolence ,vomiting ,cerebraledema,and blurring of vision.

Hyperammonemia is a metabolic disturbance characterised by an excess of ammonia in the blood. It is a dangerous condition that may lead to encephalopathy and death. It may be primary or secondary.

2. hereditary hyperammonemia:

is caused by several inborn errors of metabolism th

at are characterised by reduced activity of any of the

enzymes in the urea cycle.

1. acquired hyperammonemia :dysfunction of liver is common cause of hyperam

monemia(eg hepatic disease). porto-systemic encephalopathy: communications

between portal and systemic veins.the portal blood

may bypass the liver.

The two major types of hyperammonemia:

The major reasons of hyperammonemias:

3. Liver desfunction or porto-systemic encephalopathy,haemorrhage into GI tract.

2. Kidney secretion : kidney desfunction Degr

adation of urea in the intestine

1. Excessive putrefaction in the intestine, example:

hemorrhage of digestive tract.

hepatic encephalopathy

Hepatic encephalopathy is the occurrence of confusion, altered level of consciousness and coma as a result of excessive blood ammonia. it is also called hepatic coma or coma hepaticum. It may ultimately lead to death.

Postulated mechanisms for toxicity of high [ammonia]:

1. Depletion of glutamate & high ammonia level would drive Glut

amate Dehydrogenase reaction to reverse:

a-ketoglutarate + NAD(P)H + NH4+ glutamate + NAD(P)+ The resulting depletion of a-ketoglutarate, an essential Krebs Cycle interme

diate, could impair energy metabolism in the brain.

2. High [NH3] would drive Glutamine Synthase:

glutamate + ATP + NH3 glutamine + ADP + Pi

This would deplete glutamate – a neurotransmitter & precursor for synthesis

of the neurotransmitter GABA.

3. [glutamine], cells swelling

4. false neurotransmitter:

phenylethylamine phenolethanolamine

tyramine β-hydroxytyramine

limiting protein intake to the amount barely adequate to supply amino acids for growth, while adding to the diet the a-keto acid analogs of essential amino acids.

Liver transplantation has also been used, since liver is the organ that carries out Urea Cycle.

Treatment of deficiency of Urea Cycle enzymes (some treatments depend on which enzyme is deficient):

2. Metabolism of -keto acid

Metabolism of -keto acid

(1) Formation of non- essential AAs

(2) Formation of glucose or lipids

(3) Provide energy

(1) Formation of non- essential AAs

a. Synthesis is from –keto acids

Alanine pyruvate

Aspartate oxaloacetate

Glutamate -Ketoglutarate

transaminationreaction

b. Synthesis by amidation

Glutamine

asparagine

Glutamateaspartate

c. proline: glutamate is converted to proline by cyclization and reduction reaxtions.

D. serine,glycine,cysteine:

3-phosphoglycerate

3-phosphopyruvate

3-phosphoserine

serine

E. tyrosine:

tyrosine phenyalanine

Amino acids of converted into ketone bodies or fat

ty acids are termed ketogenic amino acids.

Amino acids of converted into glucose are termed glucogenic amino acids.

Amino acids of converted into both glucose and ketone bodies are termed glucogenic and ketogenic amino acids.

(2) Formation of glucose or lipids

Carbon skeletons of glucogenic amino acids are degraded to:

pyruvate, or

a 4-C or 5-C intermediate of Krebs Cycle. These are precursors for gluconeogenesis.

Glucogenic amino acids:

Glucogenic amino acids are the major carbon source f

or gluconeogenesis when glucose levels are low.

They can also be catabolized for energy, or converted t

o glycogen or fatty acids for energy storage.

Glucogenic amino acids: Their carbon skeletons are degraded to pyruvate, o

r to one of the 4- or 5-carbon intermediates of TCA Cy

cle that are precursors for gluconeogenesis.

Glucogenic amino acids are the major carbon sour

ce for gluconeogenesis when glucose levels are low.

They can also be catabolized for energy or converted

to glycogen or fatty acids for energy storage.

Ketogenic amino acids: Their carbon skeletons are degraded to acetyl-C

oA or one of its precursors.

Acetyl CoA, acetoacetyl CoA and its precursor ac

etoacetate, cannot yield net production of oxaloacet

ate, the precursor for the gluconeogenesis pathway.

Carbon skeletons of ketogenic amino acids can be c

atabolized for energy in TCA Cycle, or converted to

ketone bodies or fatty acids. They cannot be convert

ed to glucose.

Classification

types amino acids

Glucogenic AAs others

Glucogenic and ketogenic AAs

Ile, Phe, Tyr, Trp, Thr

Ketogenic AAs Leu, Lys

Succinyl CoA

Fumarate

Oxaloacetate

-Ketoglutarate

Citric acid

Acetyl CoA

Pyruvate

PEP

磷酸丙糖

glucose 或糖原

α- 磷酸甘油 lipids

tryglyceride

Acetoacetyl CoA

丙氨酸半胱氨酸丝氨酸苏氨酸色氨酸

异亮氨酸亮氨酸色氨酸

天冬氨酸天冬酰胺

苯丙氨酸酪氨酸

异亮氨酸 蛋氨酸丝氨酸 苏氨酸 缬氨酸

Ketone bodies

亮氨酸 赖氨酸酪氨酸 色氨酸 苯丙氨酸

谷氨酸

精氨酸 谷氨酰胺组氨酸 缬氨酸

CO2

CO2

T A C

目 录

Ketogenic amino acidsGlucogenic amino aicds

Section 5 Metabolism of Specific Amino Acid

Decarboxylation of amino acids

Metabolism of one carbon unit

Metabolism of sulfur-containing AAs

Metabolism of aromatic AAs

Metabolism of branched-chain AAs

§ 5.1 Decarboxylation of amino acids

Decarboxylation is the reaction by which CO2 is remove

d from the COOH group of an amino acid as a result an am

ine is formed.this is mostly a process confirned to putrefacti

on in intestines and produces amines.

1. Glu→γ-aminobutyric acid (GABA)

CO2COOH

CH2

L-Glu

L-glu decarboxylase

GABA

CH2

CH2NH2

COOH

CH2

CH2

CHNH2

COOH

GABA is known to serve as a normal regulator o

f neuronal activity being active as an inhibitor (pres

ynaptic inhibition).

2. Cys→taurine

CH2SH

CHNH2

COOH

L-Cys

CH2SO3H

CHNH2

COOH

sulfoalanine

CO2

CH2SO3H

CHNH2

taurine

sulfoalanine decarboxylase

3[O]

Taurine , constituent of bile acid taurocholic acid

3. His→histamine

NHN

CH2CHCOOH

NH2

L-His decarboxylase

L-His

NHN

CH2CH2NH2

histamine

CO2

Histamine acts as a neurotransmitter, particularly

in the hypothalamus. It acts as an anaphylactic and

inflammatory agent on being released from mast cells

in response to antigens.

4. Trp→5-hydroxytryptamine (5-HT)

(serotonin)

NH

CH2 CH COOH

NH2 NH

CH2 CH COOH

NH2

HO

Trp 5'-hydroxytryptophan

decarboxylaseCO2

NH

CH2 CH2 NH2HO

5'-hydroxytryptamine

Tryptophanhydroxylase

5. Polyamines

COOH

CH

(CH2)3

NH2

NH2

Ornithine

CO2NH2

(CH2)4

NH2

putrescine

S

(CH2)3

NH2

adenosine

CH3

S

adenosine

CH3

NH

(CH2)4

NH2

NH2

(CH2)3

S

(CH2)3

NH2

adenosine

CH3

S

adenosine

CH3 NH

(CH2)4

HN

NH2

(CH2)3

(CH2)3

NH2spermidine

spermine

SAM

CO2

Polyamine oxidase

β-amino-propionaldehydeO2

H2O2

Spermine spermidine

CO2+ NH4+ putrescine

Polyamine oxidase

β-amino-propionaldehyde

H2O2

Major portions of putrescine and spermidine are excreted

in urine after acetylation as acetylated derivatives.

Functions of polyamines

They have been implicated in diverse physiological processes and are involved in cell proliferation and growth. putrescine is best “marker” for cell proliferation. They are required as growth factors for cultured mammalian and bacterial cells.

Polyamines also exert diverse effects on protein synthesis. They act as inhibitors of enzymes that include protein kinase.

Spermidine has been claimed to be best “marker” of tumor cell destruction. Increased polyamine excretion has been claimed to be characteristic of maglignant diseases.

Ranges of normal excretion of polyamines: ( In urine )

putrescine: 2.7 ±0.5mg

spermine: 3.4 ± 0.7mg

spermidine: 3.1±0.6mg

§ 5.2 Metabolism of one carbon unit

1. One carbon unit

One carbon units (or groups) are one carbon-containing groups produced in catabolism of some amino acids. They are

CH3 CH2 CH CHO CH NH

methyl methylene methenyl formyl formimino

Attention: CO2 is not one carbon unit.

2. Tetrahydrofolic acid (FH4)

One carbon units are carried by FH4.

The N5 and N10 of FH4 participate in the transfer of one carbon units.

NH

HNN

N

H2N

CH2 HN

12

34 5

6

78

9 10

OH

CO NHHC

COOH

CH2 CH2 COOH

• the formation of FH4 carried one carbon unit

N5—CH3—FH4

N5 、 N10—CH2—FH4

N5 、 N10=CH—FH4

N10—CHO—FH4

N5—CH=NH—FH4

3. Formation of one carbon unit

(1) Ser→N5,N10-CH2-FH4

CH2

CHNH2

COOH+ FH4

Ser hydroxymethyl transferase

H2O

N5, N10-CH2-FH4 +CH2NH2

COOH

Ser Gly

(2) Gly→N5,N10-CH2-FH4

+ FH4Gly lyase

N5, N10-CH2-FH4 + CO2

CH2NH2

COOH + NH3

NAD+NADH+H+

Gly

Ser, Gly

(3) His →N5-CH=NHFH4

NHN

CH2CHNH2COOHNH3

NHN

CH=CHCOOH

2H2O

NHNCH=CHCOOHHOOC-CH

FH4

N5-CH=NHFH4

subaminomethyl transferase

CHNH2

COOH

£¨CH £©22

COOHsubaminomethyl Glu

His

Glu

(4) Trp→N10-CHOFH4

N

CH2CHNH2COOH

H

O2

NHCHO

CCHNH2COOH

O

N-formyl kynurenine

H2O

NH2

CCHNH2COOH

O

HCOOH

kynurenine

N10 -CHOFH4 synthetase

FH2+ATPADP+Pi

N10 -CHOFH4

Trp

4. One carbon unit exchange

H2O

CH2 FH4

N5 CH=NHFH4 N10 CHOFH4CH FH4

NH3

NH3

H2O

NADPH+H+

NAPD+

NADH+H+

NAD+

N5 CH3 FH4

N5,N10

N5,N10

5. Significance of one carbon unit

1. Substance for synthesis of nucleic acid.

N10 － CHOFH4 and N5,N10 － CH2 － FH4 can supply C2 and C8 of purine

2. one carbon unit connect amino acids metabolism and nucleic acids metabolism

If disorder of one carbon unit metabolism,induce some diseases.for example:megaloblasticanaemia

§ 5.3 Metabolism of sulfur-containing AAs

Methionine cysteine cystine

CH2SH

CHNH2

COOH

CH2SH

CHNH2

COOH

CH2

CHNH2

COOH

CH2

CHNH2

COOH

S SCH2

CHNH2

COOH

CH2

CHNH2

COOH

S SS CH3

CH2

CHNH2

COOH

CH2

S CH3

CH2

CHNH2

COOH

CH2

1. S-adenosyl methionine,SAM

adenosyl transferase

PPi+Pi+

Methionine ATP S-adenosyl methionine,SAM

1. Metabolism of Met

A

A

Methyl transferase

RH RH—CH3adenosyl

SAM S—adenosyl homocystein

homocystein

• SAM is the direct donor of methyl in body.

A A

Transmethylation and Met cycle

Significance (1) SAM is the direct donor of methyl in body.

Methylation can synthesize many important materials such as: choline, creatine, etc.

(2) N5-CH3FH4 is the indirect donor of methyl in the body.

(3) The free folic acid or VitB12 decrease(lack) will cause the decrease of DNA, which will lead to anemia.

megaloblasticanaemia

Formation of creatine

N

CH2

COOH

CH3

CNH2

HN

SAM

Arg

Gly

creatine

Synthesis of Creatine and Creatinine Creatine – nitrogenous organic acid - helps to supply energy to muscle.

Creatine by way of conversion to and from phosphocreatine is present and functions in all vertebrates as energy buffer system.

Keeps the ATP/ADP ratio high at subcellular places where ATP is needed.

The amount of creatinine produced is related to muscle mass.

The level of creatinine excretion (clearance rate) is a measure of renal function.

Glycine

2. Metabolism of cysteine and cystine

NH2CH

CH2

SH

COOH

cysteine

NH2CH

CH2

SH

COOH

cysteine

+NH2CH

CH2

S

COOH

NH2CH

CH2

S

COOH

2H

2H

cystine

Formation of PAPS

SO42-

ATPPPi

adenosine-5'-phosphosulfate (AMPS)

ATPADP

3'-phospho- adenosine-5'-phosphosulfate (PAPS)

SH

CH2

CH

COOH

NH2

Cys

H2S

[O]

NH3pyruvate

PAPS is the active sulfate group for a

ddition to biomolecules.

N

N N

N

O

OHH2O3PO

CH2O

NH2

3'-phosphoadenosine- 5'-phosphosulfate (PAPS)

PO

O

OH

O3S

phenylalanine tyrosine tryptophan

§ 5. 4 Metabolism of aromatic amino acids

1. Phe Tyr

Phe

Tyr

phenyl pyruvate

CatecholaminesMelaninFumarate

+ acetoacetate

phenylketonuria

albinism

alkaptonuria

1. Phe Tyr

CH2CHNH2COOH

+ O2

CH2CHNH2COOH

+

OH

H2O

tetrahydro- biopterin

dihydro- biopterin

Phe hydroxylase

NADPH+H+NADP+

PheTyr

N

N

N

N

CH-CH-CH3

OH

H2N

H

H

OH OH

5

78

6

1

3

N

N

N

N

CH-CH-CH3

OH

HNH

OH OH

5

78

6

1

3

Tetrahydrobiopterin Dihydrobiopterin

CH2 CH COOHNH2

CH2 C COOHOGlu ¦Á-keto-

glutarate phenyl pyruvatePhe

transaminase

Phe hydroxylase ↓→phenyl pyruvate in the body ↑ → phenylketonuria(PKU) → toxicity of central nervous system →developmental block of intelligence of children

Treatment: control the input of Phe

2. Tyr

★Catecholamines: Dopamine, norepinephrine, epinephrine

★ Melanin

★ Tyrosinase decrease will lead to albinism.

CH2CHNH2COOH

OH

CH2CHNH2COOH

OH

HO

CO2CH2CH2NH2

OH

HO

CH2CH2NH2

OH

HO

OH

CH2CH2NHCH3

OH

HO

OH

CH2CHNH2COOH

O

O

OO

NH

CH2CCOOH

OH

O

OH

OH

CH2COOH

dopa dopamine

dopa quinone norepine-

phrine

indole-5,6- quinone

fumarate +acetoacetate

Tyr

epinephrine

SAM

Tyr transaminase

melanin

hydroxy-phenyl-pyruvate

homogentisate

3. Trp

5-HT

One carbon unit

Nicotinic acid

Pyruvate and Acetoacetyl CoA

transamination

keto acid¦Á-

decarboxylation

-NH2

CO2

oxidation

enoyl-CoA

succinayl CoA succinayl CoA and acetyl CoA

Val Leu Ile

acyl-CoA

formation of

acetyl CoA and acetoacetyl CoA

§ 5.5 Metabolism of branched-chain AAs Leu, Ile, Val They are all essential AAs.