Overview of carbohydrate metabolism

Question 1

1. Describe the features that make a particular step in a linked enzyme pathway a “key step.”

Answer 1

A step where an important molecule changes its location
Enter or leave a cell

A step where energy is invested in a transition from 1 molecular state to another to activate the parent molecule

A rate-limiting step

Question 2

2. Describe the primary functions of glycolysis, gluconeogenesis, glycogen synthesis and breakdown.

Answer 2

Glycolysis: Generation of energy and useful chemical intermediates from the breakdown of glucose

Gluconeogenesis: Use non-carbon sources to generate glucose in order to maintain blood glucose levels during the fasting state and ensure sufficient glucose for the brain to function

Glycogen Synthesis: Glucose storage during periods of positive energy balance (right after eating a big bowl of pasta!) for rapidly available source of glucose

Glycogen Breakdown: Quickly mobilize glucose to meet oxidative needs when the body needs energy (such as while running a marathon!)

Question 3

3. Describe the primary functions of the TCA cycle and the electron transport system.

Answer 3

It takes place in the mitochondrial matrix
it is FLEXIBLE! It can take Acetyl CoA generated from fat, glucose, or protein metabolism and completely oxidize it to carbon dioxide while releasing a lot of energy (stored in GTP, NADH, and FADH2).

Protein can enter the cycle as α-ketoglutarate or as succinate

Alanine enters as Acetyl CoA via pyruvate
Fat enters as Acetyl CoA
Carbs enter as Acetyl CoA via pyruvate
High energy intermediates NADH and FADH2 are sent on to the electron transport chain, where they donate electrons that flow through to oxygen.

The electron transport system is where you cash in on your investment. 85% of your ATP generated from oxidizing glucose –>CO2 is derived here. It is located on the inner mitochondrial membrane and couples (really tightly) oxygen consumption with making ATP.

Question 4

4. Describe in a general sense the flux through these pathways in liver and skeletal muscle in the fasted and fed states.

Answer 4

Flux depends on a few key points:

Amount of substrate available
Lots of glucose hanging around (fed state) = favor glycolysis

Levels of key enzymes available

Allosteric Regulation-A separate molecule binds to a key enzyme to alter its activity Usually this molecule is a product of that reaction or an intermediate in an opposing reaction

Covalent modification of a key enzyme