The Cori Cycle, the Citric Acid Cycle and pyruvate dehydrogenase

lecture 4

Under what conditions does the citric acid cycle occur?

oxidative conditions in mitochondria

What is generated by the cycle for each turn?

3 NADH

1 FADH₂

2 CO₂

stage 1 of the CAC

acetyl-CoA + oxaloacetate —→ citrate + CoA

readily breakable thioacyl bond broken

aldol condensation followed by hydrolysis

stage 2 of the CAC

citrate —→ cis-aconitate —→ iso-citrate

rearrangement

3rd stage of the CAC

iso-citrate —→ alpha-ketoglutarate + CO₂

oxidation & decarboxylation

generates NADH

4th stage of the CAC

alpha-ketoglutarate + CoASH —→ succinyl-CoA + CO₂

generates NADH

5th stage of the CAC

succinyl-CoA + GDP —→ succinate + GTP + CoA-SH

generation of high energy phosphate

GTP + ADP —→ GDP + ATP

6th stage of the CAC

succinate —→ fumarate

generation of FADH₂

7th stage of the CAC

fumarate —→ malate

hydration

8th stage of the CAC

malate —→ oxaloacetate

generates NADH

return to start of cycle

anaplerosis

lose carbon from the cycle if used to generate new compounds

anapleurotic (filling up) pathway needed as well as catabolic CAC pathway

e.g. pyruvate carboxylase

pyruvate + CO₂ + ATP + H₂O —→ oxaloacetate + ADP + P_i + 2H⁺

how many ATP molecules per glucose from aerobic glycolysis & citric acid cycle?

30 ATP

pyruvate dehydrogenase

possesses 2 regulatory enzymes - PDH kinase deactivates PDH & PDH phosphatase activates PDH

PDH kinase

inhibited by pyruvate

ensures PDH is on when lots of pyruvate

PDH phosphatase

activated by calcium in skeletal muscle & insulin in adipocytes

Stimulates PDH during exercise & feeding

Direct allosteric controls on PDH

citrate synthase

isocitrate dehydrogenase

alpha-ketoglutarate dehydrogenase

citrate synthase

allosterically inhibited by ATP

regulation important for gluconeogenesis

inhibition results in diversion of oxaloacetate to gluconeogenesis while acetyl-CoA is used to make ketone bodies

both products can be used as brain fuel

isocitrate dehydrogenase (ICDH)

inhibited by high NADH/NAD ratio in fed state

stimulated by ADP & inhibited by ATP

alpha-ketoglutarate dehydrogenase

inhibited by products succinyl-CoA and NADH

stimulated by Ca²⁺

Glycolysis in cancer and the Warburg effect

running glycolysis at a higher flux promotes flux through the pentose phosphate pathway (PPP)

produces ribose for nucleotide synthesis & NADPH for fatty acid synthesis & glutathione reduction

these substrates are used to make more DAN & lipids for cell membranes & reduces effect of reactive oxygen species

allows the cancer to replicate quickly

gluconeogenesis and type II diabetes

excess of lactate, alanine and glycerol produced by adipose tissue & skeletal muscle

serve as substrates for gluconeogenesis

expression of PEP-CK increases, so more glucose is produced

hyperglycaemia

glycogen storage diseases

von Gierke’s disease

McArdle’s disease

Hers disease

von Gierke’s disease (liver)

defective glucose-6-phosphate

chronic hunger, severe hypoglycaemia, short stature, doll-like face

McArdle’s disease (muscle)

glycogen phosphorylase deficiency

intolerance to exercise & second wind syndrome

Hers disease (liver)

glycogen phosphorylase deficiency

mild symptoms, enlarged livers, hypoglycaemia

PDHC deficiency

arises from mutations in a number of genes

PDH E1 alpha - mental retardation, hypotonia, structural abnormalities in brain, microcephaly

PDH phosphatase deficiency - pyruvate dehydrogenase always inactive. glucose —→ lactic acid therefore constant lactic acidosis

neurodegenerative disorders

alzeheimer’s, parkinsons, amyotrophic lateral sclerosis

reduction in rate of glycolysis & glucose uptake

ischaemia

cutting off blood supply to tissues

e.g. stroke

increased glycolysis to keep ATP high, but increases lactate increases tissue damage.