Aminoacids1

Food

Why food?– Our body performs many processes and has

necessary infra-structure for this performance– Energy is needed to

Perform these processes Build necessary infra-structure Sustain the molecular organization

– Food provides this much needed energy

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What is food Food consists of six basic ingredients

– Carbohydrates– Lipids– Proteins– Vitamines– Minerals– Water

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Carbohydrates

Include sugars and sugar polymers (starch and glycogen) etc.– Generally used for generation of energy– Some role in structure

Polymeric forms undergo digestion

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Lipids

Include oils and fats etc– Second most preferred source of energy after

carbohydrates– Some structural role e.g. biomembranes

Undergo digestion

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Proteins

Undergo digestion to split into aminoacids The least preferred role in energy generation Perform a large number of roles, however,

major role in structure and catalysis

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Vitamins

Cofactors for many enzymes No role as energy substrate No digestion (directly absorbed)

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Minerals

Major role chemical reactions particularly oxidation/reduction reactions

No digestion, directly absorbed

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Water

Serve as solvent No digestion, directly absorbed

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Proteins

Proteins are made up of 20 different types of aminoacids

Dietary proteins are digested to yield aminoacids These aminoacids are absorbed by the intestine and

transferred to blood stream Aminoacids enter into different body cells from blood

circulation In cells, aminoacids are then bonded together to

from specific proteins (tissue /species specific) These proteins then perform various but important

functions of the cell/body

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Aminoacids

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Types of Aminoacids

Protein & Non-protein aminoacids(Standard & Non-standard aminoacids)

Protein Aminoacids are further classified on the basis of – The nature of their side chains

Polar or non-polar Aromatic or non-aromatic Function groups (hydroxyl, thio, methyl-thio) Capability of Synthesis (Essential or Non-essential)

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Definition

Organic compounds containing an amino and a carboxylic acid

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Building Blocks of Proteins

Amino acids are the building blocks of proteins

There are 20 different aminoacids used for proteins synthesis in all living organisms

An α-amino acid consists of a – central carbon atom, called the α carbon,

linked to – an amino group, the α amino group– a carboxylic acid group, the α carboxylic

group– a hydrogen atom, and – a distinctive R group called the side chain.

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Amino acid Isomers

• With four different groups α -amino acids are chiral

(except glycine)− forms two mirror-image forms the L isomer and the D isomer

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Ionization of Amino Acids

Only L amino acids are used in proteins

Aminoacids are ionized in solution

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Ionization of Amino Acids

At acidic pH– Amino group is protonated (NH3

+)

– Carboxyl group is not deprotonated (COOH)

At neutral pH– Amino group is protonated (NH3

+)

– Carboxyl group is deprotonated (COO_)

At basic pH– Amino group is not protonated (NH2)

– Carboxyl group is deprotonated (COO_)

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General Formula of an Amino Acid

R= -H, -CH2, -CH2OH, C2H4OH etc

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Side Chain Variations

Size Shape Charge hydrogen-bonding capacity hydrophobic character chemical reactivity

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Classification of Amino Acids

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Uncharged (Non-Polar) Amino Acids

Uncharged (10)

Achiral (1)

GChiral (9)

Simple Chain(4)

AVLI

Heterocyclic (1)

P

Aromatic (2)

Phe T

Sulfur Cont. (2)

CM

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Glycine

Simplest amino acid Side chain only a H atom A chiral (optically inactive)

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Physiological Roles-Glycine

Part of tripeptide coenzyme “Glutathione”, which protects –SH group from oxidation

Takes part in synthesis of heme, purines and creatin Detoxication of benzoic acid to make a soluble conjugate, the

hippuric acid Formation of bile salts by conjugation to cholic acid (glyocholic

acid and glycochenodeoxycholic acid) Can be converted to other aminoacids e.g. serine, which may be

converted to pyruvate (glucogenic aminoacid) Glycine oxidase conver glycine into glyoxalic acid, which is

oxidized to form formic acid and oxalic acid– Formic acid becomes a part of 1-carbon metabolism– Oxalic acid is excreted in urine

Excess formation leads to “Hyperoxalurea” resulting in formation of Ca-oxalate which precipitates in urinary tract

Precipitation results in Urinary Calculi and Calcification of kidneys.

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Non-Polar Amino Acids

Progressively larger side chains Isoleucine contains an additional chiral center Stabilize protein structure in aqueous solutions

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Physiological Roles-Valine

Undergoes – transamination followed by – decarboxylation

This results in the formation of – isobytyryl-CoA ultimately converted into– Succenyl-Co-A, an intermediate of TCA

cycle

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Physiological Roles-Leucine

Undergoes oxidative transamination to ultimately form

– HMG-CoA

HMG-CoA may be converted into– Cholysterol– Acetoacetate/Acetyl-CoA (so a

ketogenic aminoacid)

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Physiological Roles-Isoleucine

Undergoes oxidative transamination to form

– HMG-CoA

HMG-CoA may be converted into– Cholysterol– Acetoacetate/Acetyl-CoA (so a

ketogenic aminoacid)

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Proline

Imino acid Heterocyclic: contains a pyrrole ring

(pyrrolidine derivative) Side chain bonded to α amino group Causes bends in protein structure May form 4 hydroxyproline as a result

of post-transcriptional modification perhaps only in collagen

A small proportion may also occur as 3-hydroxyproline too

Interchangeable with ornithine, thus it can contribute to urea cycle

Can give rise to glutamate

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Sulfur Containing Amino Acids

Met is always first amino acid of a nascent protein Cys may be involved in forming disulfide bridges

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Sulfur Containing Amino Acids

A disulfide bridges between to cysteines to form a Cytine

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Aromatic Non-polar Amino Acids

Phe is purely hydrophobic but Try is less so Strongly absorb UV light (Amax 280λ)

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Phenylketonurea

It is an autosomal recessive disorder A disorder caused due to deficiency of phenylalanine

hydroxylase Frequency 1 in 20,000 Blockage of conversion to tyrosine results in ~20 fold increase

in Phe. Concentration of phenylpyruvate increases resulting in

excretion in urine Addition of ferric chloride to urine turns it olive green Phenylketonuric are severally mentally retarded, if not properly

treated Low Phe diet is solution to the problem

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Classification of Polar Amino Acids

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Charged Amino Acids

Charged (10)

Polar (5)Basic (3)

Arg His Lys

Acidic (2)

Asp Glu

Hydroxyl group (3)Amide group (2)

Asn Gln

Aliphatic (2)

Ser Thr

Aromatic (1)

Tyr

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Polar aminoacids

Aminoacids containing hydroxyl group May be post-transcriptionally phosphorylated Ser/Thr and Tyr phophorylation are very important in

Cellular signaling cell cycle regulation and tumor development

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Serine

Precursor for the synthesis of cysteine, choline and cephalins

Takes part in the synthesis of nucleic acid bases

Can be converted into glycine and pyruvic acid

Serves as carrier of phosphorus in phosphoproteins

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Threonine

Can be converted into glycine Can be converted in propionyl-CoA and then

to succinyl-CoA Serves as carrier of phosphorus in

phosphoproteins

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Tyrosine

Obtained from phenylalanine Can be converted to dihydroxyphenylalanine

(DOPA) and Dopamine Dopamine is precursor of catecholamines

(adrenaline and nor-adrenaline) Tyrosine is also a precursor for T3 and T4 Skin pigment, melanin is also a produce to

tyrosine metabolism

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Basic Aminoacids

Aminoacid group of these aminoacids gets ionized in acidic pH range

They may make part of active site Basic aminoacids include lysine, argenine

and histidine

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Lysine

It is a basic aminoacid It is among essential aminoacids It does not allow α-helix to be

formed/continued

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Argenine

Its hydrolysis yields urea Takes part in urea cycle Contributes in the formation of creatine An essential aminoacid Does not allow formation of α-helix

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Histidine

Contains an imidazole ring Near neutral pH the imidazole ring gets

charged It is often found in the active site of enzymes Imidazole ring can act as electron

acceptor/donor in an enzyme catalyzed reactions

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Acidic aminoacids

These aminoacids contain an additional –COOH group

They get ionized in the basic pH range Their side chains may act as proton acceptor They may make part of active site of an enzyme They may accept an amino group to become

amides i.e. asparagine and glutamine Glutamine play important role in nitrogen

transport/urea cycle

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Essential Aminoacids

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Aminoacids in special sources

Non-protein aminoacids May be a part of some molecules Play important role in physiological functions

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1,2. Citruline and ornithine

Found in liver Intermediates of urea cycle Take part in conversion of NH3 to urea

H2N-(CH2)3-CH-COOH

C=O Ornithine

Citrulline

NH2

H-N-(CH2)3-CH-COOH

NH2

NH2

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3. β-alanine

Part of vitamin B (pantothenic acid)

H2N-CH2-CH2-COOH

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4. Pantothenic acid

A widely distributed vitamin Make a part of co-enzyme A (Co-A) Take part in a large number of metabolic reactions

CH2—C—CH—C—NH—CH2—CH2—COOH

OH OHCH3

CH3 O

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5. γ-aminobutyric acid (GABA)

GABA is a neurotransmitter Found in nervous tissue

H2N-CH2-CH2-CH2-COOH

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6. Dihydroxyphenylalanine (DOPA)

A metabolite of phenylalanine and tyrosine

L-DOPA is used in treatment of Parkinsons Disease

OH

OH

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7. Homocystine

Formed by de-methylation of methionine

H

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8. Iodinated aminoacids

Mono-iodotyrosine (MIT) and Di-iodotyrosine (DIT) are intermediates in thyroxin synthesis

Tri-iodothyronine (T-3) and Tetra-iodothyronine (T4) are thyroxins (thyroid hormones)

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The End