http://www.youtube.com/watch?v=A
1DjTM1qnPM

After the conversion of glucose into two moles of pyruvate through the glycolysis, pyruvate is oxidatively decarboxylated into
Acetyl-CoA in the mitochondria by the pyruvate dehydrogenase enzyme complex.

Krebs' Cycle
= Citric acid cycle
= Tricarboxylic acid cycle (TCA)

1. Formation of citrate

2. Formation of Isocitrate via cis-Aconitate

3. Oxidation of Isocitrate to α-Ketoglutarate and CO
2

4. Oxidation of Isocitrate to α-Ketoglutarate and CO
2

5. Conversion of Succinyl-CoA to Succinate

6. Oxidation of Succinate to Fumarate

7. Hydration of Fumarate to Malate

8. Oxidation of Malate to Oxaloacetate

http://www.youtube.com/watch?v=A
1DjTM1qnPM

Biological importance of Krebs' cycle

Bioenergetics of Krebs' cycle

Oxidative decarboxylation of the two mole of pyruvate produced from one mole of glucose gives 2 NADH.H
+ that gives

6 ATP.
Oxidation of isocitrate by isocitrate dehydrogenase gives 2 NADH.H+ that gives

6
ATP.
Oxidative decarboxylation of

-ketoglutarate to succinyl-CoA gives 2 NADH.H+ that gives

6
ATP.

Substrate level phosphorylation from succinyl-CoA gives

2 ATP.
Oxidation of succinate to fumarate gives 2 FADH
2 thus

4 ATP.
,
Oxidation of malate to oxaloacetate gives 2 NADH.H
+ , i.e., 6 ATP.
Thus, for each mole of glucose oxidized by oxidative decarboxylation followed by Krebs' cycle 30 ATP are produced.
Complete oxidation of one glucose molecule in aerobic conditions gives 8 ATP at aerobic glycolysis +
30 ATP at Krebs' cycle giving a total of 38 ATP.

Regulation occurs at the following sites:

Citrate synthase: it is an allosteric enzyme inhibited by ATP and long chain fatty acyl-CoA. It is competitively inhibited by succinyl-CoA.
Isocitrate dehydrogenase:
The enzyme is allosterically activated by ADP and NAD and inhibited by ATP and NADH.H+.


-Ketoglutarate dehydrogenase:
The enzyme is regulated by phosphorylation
/dephosphorylation mechanism in a manner similar to pyruvate dehydrogenase. This enzyme is inhibited by accumulation of ATP, succinyl-CoA and NADH.H
+ .
Succinate dehydrogenase
It is inhibited by oxaloacetate and malonate.
That depends on the NADH/NAD ratio.

Inhibitors of Krebs' cycle:

Fluoroacetate: This compound in the form of fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate that inhibits aconitase leading to accumulation of citrate.
Arsenate: inhibits both pyruvate dehydrogenase and

-ketoglutarate dehydrogenase.
Malonate or oxaloacetate: Inhibits succinate dehydrogenase enzyme (competitive inhibition).
Mercury inhibits succinate dehydrogenase.

Sources and fates of oxaloacetate

A - Sources:
1. Carboxylation of pyruvate.
2. Oxidation of malate.
3. Cleavage of citrate.
4. Transamination of aspartate.
B - Fates:
1. Reduction into malate.
2. Synthesis of citrate .
3. Transamination into aspartate.
4. Conversion into phosphoenolpyruvate.

Hexose Monophosphate Pathway
(HMP)
(Or, Pentose Shunt)

Definition:
It is an alternative minor pathway for glucose oxidation that does not produce ATP nor utilize it.
It aims at producing NADPH+H + and ribose.
It is considered as a shunt from the main stream of glycolysis.
Intracellular site and tissue distribution:
It is cytosolic in tissues characterized by active fatty acid or steroid synthesis namely: liver, adipose tissues, lactating mammary gland, RBCs, suprarenal cortex, thyroid and testis.
It is not active in non-lactating mammary gland, and in skeletal muscles.

Regulation of HMP shunt: -
The key regulatory enzymes are glucose-6phosphate and 6-phospho-gluconate dehydrogenases.
They are activated by fed state, glucose, insulin, thyroxine and NADP but are inhibited during starvation, diabetes mellitus and with high
NADPH.H
+ /NADP ratio.

Functions and metabolic importance of HMP shunt:

I- Production of pentoses:
Tissues must satisfy their own requirement of pentoses since dietary pentoses are not utilizable and ribose is not a significant constituent of systemic blood.
Pentoses are used for:
1. Nucleic acids, ribose for RNA and deoxyribose for
DNA.
2. Coenzymes synthesis, e.g., NAD, FAD, CoASH.
3. Other free nucleotide Coenzymes, e.g., ATP, GTP,
4. Synthesis of certain vitamins, e.g., B
2 and B
12
.

II- Production of NADPH.H
+
Tissues having the following metabolic and synthetic pathways have active HMP shunt
(liver, adipose tissue, lactating mammary gland, kidney, testis, ovary and adrenal cortex).

HMP pathway is the major human source for production of NADPH.H
+ required for:
1. Fatty acid synthesis (lipogenesis) and fatty acid desaturation.
2. Cholesterol synthesis.
3. Other steroid synthesis.
4. Synthesis of sphingosine and cerebrosides.
5. Synthesis of non-essential amino acids, e.g., glutamate (through the reversible glutamate dehydrogenase) and tyrosine from phenylalanine.
6. Regeneration of reduced glutathione.
7. Metabolic hydroxylation with cytp
450
.

Favism

Glucose-6-phosphate dehydrogenase deficiency
(sometimes also called G6PD deficiency, or favism) is a hereditary disease.
As it is linked to the X chromosome, most people who suffer from it are male.
Sufferers can not make the enzyme glucose-6phosphate dehydrogenase.
This will mean the circulation of sugar in their body is different.

G6PD catalyzes the first step in the pentose phosphate pathway, which produces NADPH.
This reductant, essential in many biosynthetic pathways, also protects cells from oxidative damage by hydrogen peroxide (H
2
O
2
) and superoxide free radicals, highly reactive oxidants generated as metabolic byproducts and through the actions of drugs such as primaquine and natural products such as divicine—the toxic ingredient of fava beans.

During normal detoxification, H
2
O
2 is converted to
H
2
O by reduced glutathione and glutathione peroxidase, and the oxidized glutathione is converted back to the reduced form by glutathione reductase and NADPH.
H
2
O
2 is also broken down to H which also requires NADPH.
2
O and O
2 by catalase,
In G6PD-deficient individuals, the NADPH production is diminished and detoxification of H
2
O
2 is inhibited.
Cellular damage results: lipid peroxidation leading to breakdown of erythrocyte membranes and oxidation of proteins and DNA.

An antimalarial drug such as primaquine is believed to act by causing oxidative stress to the parasite.
It is ironic that antimalarial drugs can cause illness through the same biochemical mechanism that provides resistance to malaria.
Divicine also acts as an antimalarial drug, and ingestion of fava beans may protect against malaria.

Uronic acid Pathway

It is another minor alternative pathway for glucose oxidation by which glucuronic acid, ascorbic acid and pentoses are obtained from glucose.
Like HMP shunt, it does not need nor generate
ATP.
Site:
In cytosol of many tissues, especially liver, kidney and intestine.

Biological importance of Uronic Acid Pathway:
1-Production of UDP-glucuronic acid, which is the metabolically active form of glucuronic acid which enters in:
• Synthesis of mucopolysaccharides.
• Detoxification by conjugation: UDP-glucuronic acid is used to detoxify steroid hormones, drugs and toxins.
• Formation of conjugated bilirubin.
2-Formation of pentoses.
3-Formation of vitamin C in plants and animal except man and guinea pigs.

Edited by
Dr/Ali H. El-Far
Lecturer of Biochemistry
Fac. of Vet. Med.
Damanhour Univ.