Overview of Carbohydrate Metabolism

Question 1

5 portions of carbohydrate metabolism

Answer 1

1. Glycolysis
2. TCA cycle
3. Gluconeogenesis
4. Glycogen synthesis and breakdown
5. The Pentose Phosphate Pathway

Question 2

An Ox Red Cat & OIL RIG

Answer 2

Anion - oxidation
Cation - reduction

Oxidation Is Loss, Reduction Is Gain

Question 3

What is the final electron acceptor in mitochondria?

Answer 3

Oxygen

Question 4

Km and Vmax

Answer 4

Km = concentration (of substrate) at which the reaction is half maximal

Vmax = Maximal rate of a reaction catalyzed by an enzyme

Question 5

3 ways to identify key steps

Answer 5

1. Important molecule changes its location (enter/exit cell)
2. Investing energy in transition of a molecule to another state
3. Rate limiting step (bottleneck, often use energy to activate

Question 6

Glycolysis

Answer 6

Purpose: generation of energy and useful chemical intermediates from the breakdown of glucose.

Key steps:

1. Glucose to G6P: this involves investment of ATP, G6P becomes “trapped” within the cell because of the charge associated with the P group
2. Fructose 6-P to Fructose 1,6 BP: this is the RLS, regulated by the enzyme PFK-1.
3. PEP to pyruvate: mediated by pyruvate kinase

Question 7

2 fates of pyruvate

Answer 7

1. Presence of oxygen & mitochondria: enters TCA cycle to be oxidized to CO2 and water via Acetyl CoA & ETC. End result is ATP and water.
2. Hypoxia/exercise/no mitochondria: converted to lactate and exported out of the cell. Regenerates NAD from NADH, which allows glycolysis to continue.

Question 8

Explain how the TCA cycle is “flexible”

Answer 8

In the presence of oxygen and mitochondria, the TCA cycle enables the complete oxidation of acetyl-CoA to produce GTP, NADH, and FADH2.

In energy surplus the acetyl-CoA can leave TCA without being oxidized

In energy deficit, acetyl-CoA can continue around and release energy from the conversion of NAD and FAD to NADH and FADH2.

Alternatively, pyruvate from lactate or AA metabolism can enter as oxaloacetate and leave to join gluconeogenesis

Question 9

Describe the pathway from lactate to PEP in gluconeogenesis

Answer 9

Lactate –> Pyruvate –> TCA cycle as OAA –(PEPCK)–> PEP

Question 10

Gluconeogenesis

Answer 10

Purpose: glycolysis in reverse - get moar glucose.

Key steps:

1. Pyruvate –(PEPCK)–> PEP
2. Fructose 1,6 BP –(fructose 1,6 bisphosphatase)–> Fructose-6-P
3. Glucose-6-P –(glucose-6-phosphatase)–> free glucose (ONLY in liver and kidney)

Question 11

Glycogen

Answer 11

Purpose: rapidly available source of glucose for acute energy needs

Key steps:

1. Glucose-1-P –> UDP-glucose; committed step
2. UDP-glucose to growing glycogen molecule via glycogen synthase + branching enzyme

(note that the reverse enzyme is glycogen phosphorylase + debranching enzyme to go from glycogen to glucose-1-P)

Question 12

HMP Shunt

Answer 12

Purpose:

1. Make NADPH for FA biosynthesis and defense against oxidative stress
2. Generation of riboses for nucleotide synthesis

Question 13

4 ways to regulate flux through a pathway

Answer 13

1. Amount of substrate available
2. Levels/amount of key enzymes available
3. Allosteric regulation - another molecule alters the activity of a key enzyme in a pathway
4. Covalent modification of a key enzyme - mostly ends up modifying its activity; this is a common way that hormones modify flux. Can also be through changes in Km or Vmax.

Question 14

Counter-regulatory hormones

Answer 14

glucagon, catecholamines, growth hormone, cortisol

Question 15

Action of catecholamines and glucagon

Answer 15

Bind to membrane associated receptors to change levels of cAMP and protein kinases, which in turn alter key enzyme activities.

Fed state: insulin high; counter regulatory hormones low