the roles of ATP in living cells 2

Question 1

describe the crossroads of pyruvate in metabolism? 4

Answer 1

• Lactate (lactate dehydrogenase)
• Oxaloacetate (pyruvate carboxylase)
• Alanine (alanine aminotransferase)
• Acetyl-CoA (pyruvate dehydrogenase complex)

Question 2

describe the transport of pyruvate into the mitochondria? 5

Answer 2

• Occurs via specific carrier protein embedded in the mitochondrial membrane in aerobic conditions
• Pyruvate undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex to form Acetyl CoA
• pyruvate+CoA +NAD+–> acetyl CoA +CO2 +NADH +H+
• Reaction is irreversible and it the link between glycolysis and the citric acid cycle
• this is done by the PDH complex, which consists of 3 enzymes and 5 coenzymes

Question 3

what is the tricarboxylic acid cycle? 4

Answer 3

• Citric acid/ Krebs cycle
• Final common pathway for the oxidation of fuel molecules
• In 8 steps, acetyl residues are oxidised to CO2
• Reducing equivalents transferred to NAD+ or FAD to form NADH and FADH2

Question 4

give an overview of the TCA cycle? 4

Answer 4

• A 4-carbon unit condenses with a 2-carbon unit
• Eventually 2 carbons leave the cycle as CO2 and the 4C unit is regenerated
• Involves 4 oxidation-reduction reactions (NADH and FADH2 production)
• One molecule of ATP is produced directly for each round of the cycle (shown by the GTP molecule)

Question 5

what are the 8 intermediates of the TCA cycle?

Answer 5

• citrate
• isocitrate (NADH and CO2 released) (isocitrate dehydrogenase- oxidative decarboxylation)
• alpha-ketoglutarate (NADH and CO2 released) (alpha-ketoglutarate- oxidative decarboxylation)
• succinyl CoA (GTP released)
• succinate (FADH2 released)
• furmarate
• L-malate (NADH released)
• oxaloacetate + acetyl CoA (citrate synthase- condensation)

Question 6

describe the regulation of the TCS cycle? 4

Answer 6

• Flow of carbon atoms from pyruvate into and through the TCS cycle is tightly regulated at 2 levels
• Conversion of pyruvate to acetyl-CoA (PHD reaction)
• Entry of acetyl-CoA into the TCA cycle (citrate synthase reaction)
• Also regulated at isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase reactions

Question 7

what else can be a source of acetyl-CoA? 2

Answer 7

• fatty acids

- amino acids

Question 8

why are components of the TCA cycle important? 3

Answer 8

• they are biosynthetic intermediates
• replenished by anapldrotic actions
• concentrations of TCA intermediates are held in a dynamic balance

Question 9

what are the produces to the TCA cycle?

Answer 9

Energy released from oxidations is conserved in the production of:
3 NADH, 1 FADH2, 1 GTP (ATP)
2 CO2 is also produced
This cycle operates twice for each molecule of glucose

Question 10

what happens to NADH after the TCA cycle? 7

Answer 10

• The inner mitochondrial membrane is impermeable to NADH
• There is no carrier in the membrane to transport it across
• Electrons from NADH enter the mitochondria by shuttles
• There are 2 shuttles
• The glycerol-3-phosphate shuttle, especially prevalent in brain and muscle
• The malate-aspartate shuttle, in the liver and heart
• Both shuttles act to regenerate NAD+ and make 1.2 or 2.5 moles of ATP