An Introduction to Metabolism

Question 1

Glycolysis- Stage 2 (2 steps)

Answer 1

Fructose-1,6-bisphosphate is converted into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate;

Dihydroxyacetone phosphate is converted into glyceraldehyde-3-phosphate.

Net products are 2 x glyceraldehyde-3-phosphate.

Question 1

glycolysis step 1

Answer 1

Glycolysis occurs in the cytosol;

Phase 1 activates glucose for metabolism (energy is put in as 2 x ATP) and rearrangement (allows formation of 2 x C3 sugars in stage 2);

Double phosphorylation causes ring opening of sugar.

Question 2

Glycolysis – Stage 3 (5 steps)

Answer 2

Each step is repeated twice as two molecules of glyceraldehyde-3-phosphate are generated in step 2;

Energy producing stage. Products for stage 3 are 2 x NADH and 4 x ATP

Question 3

Anaerobic respiration

Answer 3

Typically occurs in muscle during anaerobic exercise;

In the absence of oxygen, pyruvate is reduced to R-lactate (D-lactate);

This oxidises NADH to NAD+ and allows glycolysis to continue;

R-Lactate can be reoxidized to pyruvate using NAD+.

Question 4

The Cori cycle

Answer 4

Recycles R-lactate to glucose;

Lactate is transported from muscle to liver in blood;

Lactate dehydrogenase in liver converts R-lactate to pyruvate;

Pyruvate is converted to glucose by gluconeogenesis;

Process requires input of 6 x ATP per glucose molecule cf. 2 x ATP molecules from glycolysis.

Question 5

Gluconeogenesis

Answer 5

Gluconeogenesis primarily occurs in the liver cytosol;

2 x pyruvates required for each glucose;

ATP, CO2 and pyruvate make oxaloacetate (in mitochondria). Oxaloacetate is exported to the cytosol and converted into phosphoenolpyruvate;

2 x ATP and 1 x NADH also required;

Additional enzymes are required (those shown) where steps in glycolysis are irreversible;

Phosphatase allows export of glucose to other tissues

Question 6

Formation of acetyl-CoA

Answer 6

eps in the reaction:
TPP anion adds to pyruvate and CO2 is released;
Lipoamide disulfide is added to acetyl group and a redox reaction occurs;
Disulfide exchange occurs to form acetyl-CoA and reduced lipoamide;
Reduced lipoamide is oxidised to disulfide form using FAD;
FADH2 is oxidised by NADH, which is fed into the electron transport system

Question 7

The tricarboxylic acid cycle

Answer 7

The cycle is used in both catabolic and anabolic reactions;
It consists of 8 steps:
Four stages
Condensation and rearrangement (steps 1 & 2);
Decarboxylation (steps 3 & 4);
Formation of GTP (step 5);
Conversion of succinate to oxaloacetate (steps 6-8).

Acetyl-CoA is oxidised to 2 x CO2. Other products are: 3 x NADH and H+; 1 x FADH2; 1 x GTP.

O2 must be present to allow re-oxidation of reduced cofactors by electron transport system.