Biochem revision - Week 5 Glycolysis and the Krebs Cycle

Question 1

What are the products and reactants of glycolysis?

Answer 1

Products:
2 x pyruvate (3C)
4 x ATP\
2 x NADH
2 x H+

Reactants:
Glucose-6-phosphate (already converted from glycogen /isomerized)

2 ATP molecules used to phosphorylate glucose if not already completed

Question 2

How does exercise augment the glycolytic rate?

Answer 2

1. Increased substrate avalibility from glycogenolysis (glucose and glucose-1-phosphate for use in glycolysis)
2. Greater blood flow increases flow of glucose molecules to active muscle
3. Increased GLUT4 transporters in plasma membrane
4. Phosphofructokinase is inhibited by ATP and PCr concentrations but activated by increased [AMP] (third rctn/phosphorylation)
5. Pyruvate kinase is inhibited by ATP and PCr concentrations but activated by increased [ADP] (last reaction/dephosphorylation)

Question 3

What are the reactants and products of the citric acid cycle or Krebs cycle?

Answer 3

Reactants:
Acetyl coA

Products:
ATP, NADH, and FADH2

Question 4

How is acetyl coA formed prior to the Krebs cycle? Where does this occur?

Answer 4

Pyruvate is oxidized by pyruvate dehydrogenase, in turn generating 1 NADH

This occurs in the mitochondria after pyruvate has been shuttled across the membrane

*The other 2 NADH from glycolysis then pass their e- through an enzyme on the membrane to FADH2 because it is impermeable to membrane between the cytosol and the mitochondria

Question 5

Where does glycolysis occur? Where does the Krebs cycle occur? Where does ATP synthesis occur?

Answer 5

Glycolysis occurs in the cytosol

The Krebs cycle occurs in the mitochondrial matrix

ATP synthesis occurs in the intermembrane space (btw the outer membrane of the mitochondria and the cristae) and the matrix

Question 6

What is the main goal of the Krebs cycle?

Answer 6

To oxidize acetyl coA into CO2 via a series of 9 reactions.

These 9 rctns produce metabolites that are then used as reducing agents in the electron transport chain (NADH and FADH2)

Question 7

What are the reactants and products of the Citric Acid Cycle?

Answer 7

Net gain for 2x acetyl coA:

Reactants:
2x Acetyl coA

Products:
2 ATP
6 NADH
2 FADH2
8 H+

*Similar to glycolysis, the Krebs cycle can be upregulated with increase in ADP/Ca2+ and decrease in ATP

Question 8

What are the reactants and products of oxidative phosphorlyation?

Answer 8

Net amounts:

Reactants:
8 NADH and 4 FADH2

Products:
26 ATP
6CO2
6H2O

Question 9

How does the ETC create a flow of protons across the membrane into the intermembrane space?

Answer 9

The ETC re-oxidizes NADH and FADH2 to their oxidized forms (NAD+ and FADH), releasing 2 e- per proton lost.

e- are then passed down a series of e- carriers (proteins) imbedded in the membrane, each with a higher e- affinity than the last

O2 is the final e- acceptor and H2O is produced.

NAD+ and FADH are recycled back into the cytosol and mito. matrix

Question 10

How many H+ are moved into the intermembrane space per 2e- (The # released from the oxidation of one FADH2 or NADH)?

Answer 10

A total of 10 H+ are ejected from the matrix per 2 e- transferred from NADH to O2, but only 6 H+ per FADH2 because it enters at complex 2)

There are four complexes. C1 and C4 move 4 protons, C3 moves 2 protons C2 moves none

Question 11

What ATP synthase do? What direction does this gradient flow in?

Answer 11

The enzyme binds ADP + Pi when 4 protons are present, yielding 1 ATP molecule

The protons flow through ATP synthase from the intermembrane space to the mitochondrial matrix

Question 12

When is anerobic metabolism (lactate dehydrogenase) utilized?

Answer 12

When the ratio of NADH:NAD+ increases to a point where the ETC can’t keep up, thus these metabolites cannot be shuttled back into the cytosol quick enough for glycolysis

Lactate dehydrogenase picks up the slack by converting into lactate and regenerating NAD+ at a high rate.

Pyruvate + NADH + H+ = Lactate + NAD+

Question 13

What can happen to lactate once it is formed?

Answer 13

a) Lactate leaves the working muscle into the blood stream and is then re-oxidized into pyruvate in less active muscle fibers via the reversal rctn with pyruvate dehydrogenase to yield 14 ATP via the Krebs cycle and Oxi. Phos.

b) Lactate can re-oxidize into pyruvate in the liver and then, through gluconeogenesis, be converted back into glucose and shuttled back to the working muscle (this is known as the Cori Cycle).

Question 14

What happens when a substrate (NADH or FADH2) is oxidized?

Answer 14

These substrates give up a hydrogen cation, and 2 e- each

Question 15

What are the two hormones that regulate blood glucose? Where do they come from and what do they do?

Answer 15

Insulin: secreted by B-cells in the pancreas in response to high levels of blood glucose

Stimulates glycogenesis and glucose uptake by cells

Glucagon: secreted from the a-cells of the pancreas in response to low blood glucose

Stimulates glycogenolysis

Question 16

What is the difference btw type 1 and type 2 diabetes?

Answer 16

Type 1: Impaired insulin secretion (autoimmune disease where T-cells target the destruction of the pancreas)

Type 2: Impaired sensitivity to insulin (desensitized receptors)

Question 17

When can hypoglycemia occur in a diabetic taking insulin?

Answer 17

If insulin is injected before exercise, blood glucose levels can drop significantly

Question 18

How can regular exercise affect glucose uptake?

Answer 18

Aerobic exercise increases GLUT4
expression in muscle (T1 fibers) and therefore increases glucose uptake

Exercise can also improve insulin sensitivity

Question 19

What is Oxidative Phosphorylation?

Answer 19

The spontaneous flow of e- through each of the haem protein complexes and then to oxygen to reduce it, coupled with the ejection of H+ from the matrix into the intermembrane space.

Question 20

How/why are H+ protons produced in the TCA cycle?

Answer 20

Because as each carbon is removed from acetyl coA, one H- ion is released and reacted with NAD+/FADH to produce NADH/FADH2, and one H+ proton is released to add to the acidic environment

Question 21

What are the roles of Phosphofructokinase and Pyruvate Kinase? How are they affected?

Answer 21

Both are enzymes that serve in glycolysis.

Phosphofructokinase is the first enzyme that phosphorylates glucose-6-phosphate. It is activated by increased [AMP] and decreased [ATP] and [PCr]

Pyruvate Kinase is the last enzyme in glycolysis that dephosphorylates phosphoenolpyruvate into pyruvate. It is activated by increased [ADP] and decreased [ATP] and [PCr]