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February 15 1 Dentistry Medicine
February 15 2 Medicine
Pyruvate Dehydrogenase
Pyruvate
dehydrogenase Dihydrolipoyl
dehdrgenase
Dihydrolipoyl
transacetylase
TPP FAD Lipoic
February 15 3
irreversible;
in mitochodria.
Medicine
Pyruvate dehydrogenase complex:
E1 pyruvate dehydrogenase
Es E2 dihydrolipoyl transacetylase
E3 dihydrolipoyl dehydrogenase
Thiamine pyrophosphate, TPP (VB1)
Cofactors Lipoic Acid
(Vitamins) CoA (pantothenic acid)
FAD (VB2)
NAD+ (VB3)
Tender
Loving
Care
For
Nancy
Medicine
mnemonics
Pyruvate Dehydrogenase
Subunits
Enzyme Abbreviated Prosthetic Group
Pyruvate
Dehydrogenase E1
Thiamine
pyrophosphate (TPP)
Dihydrolipoyl
Transacetylase E2
Lipoamide
Dihydrolipoyl
Dehydrogenase E3
FAD
Medicine
February 15 7 Medicine
The lipoyllysyl moiety is the prosthetic group of dihydrolipoyl transacetylase (E2 of the
PDH complex). The lipoyl group occurs in oxidized (disulfide) and reduced (dithiol)
forms and acts as a carrier of both hydrogen and an acetyl (or other acyl) group.
Lipoic acid (lipoate) in amide linkage with a Lys residue
February 15 8 Medicine
Localization of pyruvate decarboxylation
The transport of pyruvate into the mitochondria is via a transport protein and is active, consuming energy. Passive diffusion of pyruvate into the mitochondria is impossible because it carries a negative charge.
On entry to the mitochondria the pyruvate decarboxylation occurs, producing acetyl CoA. This irreversible reaction traps the acetyl CoA within the mitochondria (the acetyl-CoA can only be transported out of the mitochondrial matrix under conditions of high oxaloacetate via the citrate shuttle, a TCA intermediate). The carbon dioxide produced by this reaction is nonpolar and small, and can diffuse out of the mitochondria and out of the cell.
Medicine
Citrate Shuttle Acetyl-CoA can only be transported out of the mitochondrial matrix under conditions of high
oxaloacetate via the citrate shuttle
Medicine
Oxidative decarboxylation of pyruvate to acetyl-CoA by the PDH complex
The first step is the slowest and therefore limits the rate of the overall reaction.
It is also the point at which the PDH complex exercises its substrate specificity February 15 13 Medicine
Medicine
Oxidative decarboxylation of
pyruvate.
Regulation of Pyruvate
Dehydrogenase Complex:
NADH competes with NAD+ for binding to E3.
Acetyl CoA competes with CoA for binding to E2.
Pyruvate (-)
Pyruvate dehydrogenase is inhibited when one or more of the three following ratios are increased: ATP/ADP, NADH/NAD+ and acetyl-CoA/CoA.
In eukaryotes PDC is tightly regulated by its own specific pyruvate dehydrogenase kinase (PDK) and pyruvate dehydrogenase phosphatase (PDP), deactivating and activating it respectively.
PDK phosphorylates three specific serine residues on E1 with different affinities. Phosphorylation of any one of them renders E1 (and in consequence the entire complex) inactive.
Dephosphorylation of E1 by PDP reinstates complex activity. Products of the reaction act as allosteric inhibitors of the PDC, because
they activate PDK. Substrates in turn inhibit PDK, and thus, reactivating PDC.
During starvation, PDK increases in amount in most tissues, including skeletal muscle, via increased gene transcription. Under the same conditions, the amount of PDP decreases. The resulting inhibition of PDC prevents muscle and other tissues from catabolizing glucose and gluconeogenesis precursors. Metabolism shifts toward fat utilization, while muscle protein breakdown to supply gluconeogenesis precursors is minimized, and available glucose is spared for use by the brain.
Calcium ion has a role in regulation of PDC in muscle tissue, because it activates PDP, stimulating glycolysis on its release into the cytosol - during muscle contraction.
Medicine
(1) Pyruvate dehydrogenase
complex
Pyruvate dehydrogenase(active form)
allosteric inhibitors:
ATP, acetyl CoA,NADH, FA
allosteric activators:
AMP, CoA,
NAD+,Ca2+
pyruvate dehydrogenase (inactive form)
P
pyruvate dehydrogenase kinase
pyruvate dehydrogenase phosphatase
ATP
ADPH2O
Pi
Ca2+,insulin acetyl CoA,NADH
ADP,
NAD+Medicine
During starvation:
Pyruvate Dehydrogenase Kinase increases in amount in most tissues, including skeletal muscle, via increased gene transcription.
Under the same conditions, the amount of Pyruvate Dehydrogenase Phosphatase decreases.
The resulting inhibition of Pyruvate Dehydrogenase prevents muscle and other tissues from catabolizing glucose & gluconeogenesis precursors.
Metabolism shifts toward fat utilization.
Muscle protein breakdown to supply gluconeogenesis precursors is increased.
Available glucose is spared for use by the brain.
Medicine
Advantages of multienzyme complex:
The PDH complex is a classic multienzyme complex in which a series of chemical intermediates remain bound to the enzyme molecules as a substrate is transformed into the final product. Five cofactors, four derived from vitamins, participate in the reaction mechanism. The regulation of this enzyme complex also illustrates how a combination of covalent modification and allosteric regulation results in precisely regulated flux through a metabolic step
Higher rate of reaction: Because product of one enzyme acts as a substrate of other, and is available for the active site of next enzyme without much diffusion.
Minimum side reaction.
Coordinated control.
Medicine
Treatment Use of a ketogenic diet has been described. The ketogenic diet is a high-fat, adequate-protein, low-carbohydrate
diet. The diet mimics aspects of starvation by forcing the body to burn fats rather than carbohydrates.
Current research is being conducted on the viability of Dichloroacetic acid to treat the lactic acidosis commonly accompanied by this disorder. Salts of DCA have been studied as potential drugs because they stimulates the activity of the enzyme pyruvate dehydrogenase by inhibiting the enzyme pyruvate dehydrogenase kinase. Thus, it decreases lactate production by shifting the metabolism of pyruvate from glycolysis towards oxidation in the mitochondria.
Additionally, there is research being conducted on the viability of gene therapy for sufferers of this condition as well as many other mitochondrial defects.
Medicine
Medicine
Thiamine is involved in a vast array of functions:
Carbohydrate metabolism Production of the neurotransmitters glutamic
acid and GABA, through the citric acid cycle
Lipid metabolism, necessary for myelin production Amino acid metabolism Neuromodulation - physiological process by which a
given neuron uses one or more neurotransmitters to
regulate diverse populations of neurons.
Thiamin (Vitamin B1) deficiency causes Beriberi
Thiamine pyrophosphate (TPP) is an important cofactor of pyruvate dehydrogenase complex, or PDC a critical enzyme in glucose metabolism. Thiamine is neither synthesized nor stored in good amounts by most vertebrates. It is required in the diets of most vertebrates.
The body cannot produce thiamine and can only store up to 30 mg of it in tissues. Thiamine is mostly concentrated in the skeletal muscles. Other organs in which it is found are the brain, heart, liver, and kidneys. The half-life of thiamine is 9-18 days. It is excreted by the kidney
Thiamine deficiency ultimately causes a fatal disease called Beriberi characterized by neurological disturbances, paralysis, atrophy of limbs and cardiac failure. Note that brain exclusively uses aerobic glucose catabolism for energy and PDC is very critical for aerobic catabolism. Therefore thiamine deficiency causes severe neurological symptoms.
The main types of beriberi are:
Dry beriberi and Wernicke-Korsakoff syndrome affect the peripheral and central nervous system respectively.
Wet (edematous) beriberi affects the cardiovascular system, as well as other bodily systems. Infantile beriberi affects mostly children in developing countries
Prevalence
Beriberi is rare in developed countries because most foods are now vitamin-enriched. Excluding the presence of arsenic in the environment (e.g. well water) one can get enough thiamine by eating a normal, healthy diet. Today, beriberi occurs mostly in patients who abuse alcohol. Drinking heavily can lead to poor nutrition, and excess alcohol makes it harder for the body to absorb and store thiamine.
Medicine
Insufficient thiamine significantly impairs glucose oxidation, causing highly aerobic tissues, such as brain and cardiac muscle, to fall first. In addition, branched-chain amino acids are sources of energy in brain and muscle
Wernicke-Korsakoff syndrome is a brain disorder due to thiamine deficiency.
Wernickes Encephalopathy (peripheral neuropathy ) and Korsakoffs Psychosis are the acute and chronic phases, respectively, of the same disease.
Korsakoff syndrome, or Korsakoff psychosis, tends to develop as Wernicke's symptoms go away. Wernicke's encephalopathy causes brain damage in lower parts of the brain called the thalamus and hypothalamus. Korsakoff psychosis results from damage to areas of the brain involved with memory.
The disease is typically associated with chronic alcoholism, but may be associated with malnutrition or other conditions which cause nutritional deficiencies. Alcohol interferes with thiamine absorption from the intestine,
Congestive heart failure may be a complication (wet beri-beri) owing to inadequate ATP and accumulation of ketoacids in the cardiac muscles. (Peripheral vasodilation leading to a high cardiac output state. This leads to salt and water retention and Edema.)
Two other enzyme complexes similar to pyruvate dehydrogenase that use thiamine are:
-Ketoglutarate dehydrogenase (citric acid cycle) Branched-chain ketoacid dehydrogenase (metabolism of branched-chain amino acids)
.
Wernicke-Korsakoff Syndrome (WKS)
Medicine
Arsenic Poisoning: Arsenic compounds such as arsenite (AsO3---) organic arsenicals are poisonous because they covalently bind to sulfhydryl compounds
(SH- groups of proteins and cofactors). Dihydrolipoamide is a critical cofactor of
PDC, and it has two-SH groups, which are important for the PDC reaction. These
SH groups are covalently inactivated by arsenic compounds as shown below;
OH HS S
-O As + -O As + 2H2O
OH HS S
R R
Arsenic compounds in low doses are very toxic to microorganisms, therefore
these compounds were used for the treatment of syphilis and other diseases in
earlier days. Arsenicals were first antibiotics, but with a terrible side effects as
they are eventually very toxic to humans.
Unfortunately and ignorantly, a common nineteenth century tonic, the Fowlers solution contained 10 mg/ml arsenite. This tonic must have been responsible for
many deaths, including the death of the famous evolution scientist Charlse
Darwin.
Medicine
Medical Students
Glycolysis in disease
Genetic diseases
Glycolytic mutations are generally rare due to
importance of the metabolic pathway, this
means that the majority of occurring mutations
result in an inability for the cell to respire, and
therefore cause the death of the cell at an early
stage. However, some mutations are seen with
one notable example being Pyruvate kinase
deficiency, leading to chronic hemolytic anemia.
Medical Students
Genetic defects of this enzyme cause the disease known as pyruvate kinase deficiency.
In this condition, a lack of pyruvate kinase slows down the process of glycolysis.
This effect is especially devastating in cells that lack mitochondria, because these cells must use anaerobic glycolysis as their sole source of energy because the TCA cycle is
not available.
One example is red blood cells, which in a state of pyruvate kinase deficiency rapidly become deficient in ATP and can undergo hemolysis. Therefore, pyruvate kinase
deficiency can cause hemolytic anemia and an increase in plasma bilirubin.
A discrepancy between red blood cell energy requirements and ATP generating capacity produces irreversible membrane injury resulting in cellular distortion, rigidity,
and dehydration. This leads to premature erythrocyte destruction by the spleen and
liver.
Pyruvate Kinase Deficiency
Medical Students
The buildup of reaction intermediates can also increase the level of 2,3-bisphosphoglycerate in the cells and affect tissue oxygenation.
This will cause a "right shift" in the hemoglobin oxygen saturation
curve, implying a decreased oxygen affinity for the hemoglobin and
earlier oxygen unloading than under normal conditions.
Low prevalence, No Heinz bodies (inclusions or aggregates within red blood cells composed of denatured hemoglobin )
Second cause of hemolytic anemia after G6Pdase deficiency.
Treatment can include a blood transfusion or removal of the spleen. Treatment is usually effective in reducing the severity of the
symptoms.
Pyruvate Kinase Deficiency
Medical Students
2-3-BPG BIND TO Hb, DECREASE ITS AFFINITY TO O2, INCREASE O2
AVALABILITY TO TISSUE.
In RBC
Medical Students
o2O2 PRESSURE (torr)
SATURATION
1
0
10 50
No BPG
With BPG
BPG Lowers the binding affinity of Hb for O2
[BPG] = 0, Hb P50 = 1 torr [BPG] = 4000mM, Hb P50 = 26 torr
Without BPG, Hb couldnt unload O2 in cells
Hb
Medical Students
Glycolysis as an indicator of
disease
The glycolytic rates in malignant, rapidly-growing tumor cells are up to 200 times higher than those of their normal tissues of origin, despite the ample availability ofoxygen. A classical explanation holds that the local depletion of oxygen within the tumor is the cause of increased glycolysis in these cells. However, there is also strong experimental evidence that attributes these high rates to an over-expressed form of the enzyme hexokinase (Bustamante and Pedersen 2005),which is responsible for driving the high glycolytic activity when oxygen is not necessarily depleted. This finding currently has an important medical application: aerobic glycolysis by malignant tumors is utilized clinically to diagnose and monitor treatment responses of cancers using medical imaging techniques (Pauwels et al. 2000, PETNET Solutions 2006).
Medical Students
Ethanol Metabolism and Gluconeogenesis
Ethanol strongly inhibits gluconeogenesis and can bring about hypoglycemia, a potentially dangerous decrease in blood glucose levels. Ethanol metabolism occurs primarily in the liver. The reaction, catalyzed by alcohol dehydrogenase is shown as follows:
Ethanol + NAD+ Acetaldehyde + NADH + H+
The NADH produced in this reaction shifts the equilibrium in the liver cytosol of the lactate dehydrogenase from pyruvate formation to lactate synthesis.
The NADH also favors reduction of oxaloacetate to malate in the reaction catalyzed by malate dehydrogenase, making less oxalacetate available forgluconeogenesis. The resulting hypoglycemia can affect the part of the brain concerned with temperature regulation and the body temperature can fall by as much as 2 C. Thus, feeding alcohol to people suffering from hypothermia is counterproductive. Metabolically speaking, glucose would be far more effective in raising body temperature.
Medical Students
Hereditary fructose-
1,6-bisphosphatase
deficiency results in
severely impaired
hepatic
gluconeogenesis and
leads to episodes of
hypoglycemia, apnea,
hyperventillation,
ketosis and lactic
acidosis.
FRUCTOSE METABOLISM
Sequestering ATP
Medical Students
GALACTOSE METABOLISM
SORBITOL/
Cataracts early in life
Cataracts early in life.
Vomiting, diarrhea following lactose ingestion.
Lethargy.
Liver damage, hyperbilirubinemia.
Mental retardation.
Gal-1-P-uridyltransferase deficiency:
Sequestering ATP
Medical Students
Figure 12.4. Sorbitol metabolism
Medical Students
Conversion of glucose to fructose via sorbitol Synthesis of sorbitol:
~Aldose reductase reduces glucose, producing sorbitol (glucitol).
~This enzyme is found, in lens, retina, Schwann cells, liver, kidney,
placenta, red blood cells, and in cells of the ovaries and seminal
vesicles
~In cells of the liver, ovaries, sperm, and seminal vesicles, there is a
second enzyme, sorbitol dehydrogenase, that can oxidize the sorbitol to produce fructose sperm cells use fructose .
~pathway from sorbitol to fructose in the liver provides a mechanism
by which any available sorbitol is converted into a substrate that can
enter glycolysis or gluconeogenesis.
Medical Students
The effect of hyperglycemia on sorbitol metabolism:
Because insulin is not required for the entry of glucose into the
cells where sorbitol syn. occurs. large amounts of glucose may
enter during hyperglycemia (uncontrolled diabetes).
Elevated intracellular glucose produce a significant increase in the
amount of sorbitol, which cannot pass efficiently through cell
membranes and, therefore, remains trapped inside the cell.
This is exacerbated when sorbitol dehydrogenase is low or absent(in retina, lens, kidney, and nerve cells).
result, sorbitol accumulates in these cells(strong osmotic effects
(cataract formation, peripheral neuropathy, and vascular problems
leading to nephropathy and retinopathy)
Medical Students
2. The effect of hyperglycemia on sorbitol metabolism
- Because insulin is not required for entry of gluc into cells listed in previous paragraph, large amounts of gluc may enter these cells during times of hyperglycemia, e.g., in uncontrolled diabetes.
- Elevated intracellular gluc concs & an adequate supply of NADPH cause aldose reductase to produce a sufficient increase in the amount of sorbitol, which cant pass efficiently through CMs &, therefore, remains trapped inside cell.
- This is exacerbated when sorbitol dehydrogenase is low or absent, e.g., in retina, lens, kidney & nerve cells. As a result, sorbitol accumulates in these cells, causing strong osmotic effects &, therefore, cell swelling as a result of water retention
- Some of the pathologic alterations associated with diabetes can be attributed, in part, to this phenomenon, including cataract formation, peripheral neuropathy, & vascular problems leading to nephropathy, & retinopathy