Overview of Carbohydrate Metabolism

Question 1

What are the three features that make a particular step in a linked enzyme pathway a “key step”?

Answer 1

1 - A step where an important molecule changes location (Enters or leaves cell, enters or leaves mitochondria, etc…)

2 - A step where energy is invested (A transition from one molecular state to another to activate the parent molecule)

3 - A rate limiting step

Question 2

Describe the primary function of glycolysis

Answer 2

Generation of energy and useful chemical intermediates from the breakdown of glucose.

Question 3

Describe the primary function of Gluconeogenesis

Answer 3

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

Question 4

Describe the primary function of Glycogen Synthesis

Answer 4

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

Question 5

Describe the primary function of Glycogen Breakdown

Answer 5

Quickly mobilize glucose to meet oxidative needs when the body needs energy (Such as when running a marathon)

Question 6

What is the primary function of the Citric Acid Cycle?

Answer 6

TCA cycle occurs in the mitochondria and produces the high energy intermediates of GTP, NADH, and FADH2 which transfer electrons to the electron transport chain. Accepts proteins, fats, or carbohydrates. Proteins enter as succinate or a-ketoglutarate (alanine enters as acetyl-CoA via pyruvate), fats and carbs enter as Acetyl-CoA.

Question 7

What is the primary function of the Electron Transport Chain?

Answer 7

ETC accepts electrons from NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane. The electrons flow through to oxygen, the final e- acceptor. The proton gradient is used to generate ATP, thus O2 consumption is very tightly coupled to ATP generation. 85% of all ATP generated by the breakdown of glucose is generated in the ETC.

Question 8

Describe how the flux through glycolysis and gluconeogenesis pathways changes in fed vs unfed states

Answer 8

Flux depends on four primary factors:
1- Amount of Substrate Available (Fed = lots of glucose = favors glycolysis)

2 - Levels of Key Enzymes Available (PEPCK is a key enzyme in gluconeogenesis, insulin decreases transcription of PEPCK, thus Fed state inhibits gluconeogenesis)

3 - Allosteric Regulation (A separate molecule binds to a key enzyme to alter its activity, usually a product of that reaction or an intermediate of the opposing reaction; NADPH/NADP+ inhibit G6PD to diminish PPP)

4 - Covalent Modification of a Key Enzyme (Usually adding or removing phosphate groups, frequently seen in glycogen synthesis/breakdown)