Chapter 12: Carbohydrate Metabolism

Summarize each of the following processes:

• Glycolysis

• Gluconeogenesis

• Glycogen Synthesis & Breakdown

• Breakdown of glucose → pyruvate for energy

• Combine smaller C compounds to make glucose

• Storage or recovery of glucose from glycogen polymer

What is the net yield of glycolysis?

• 2 pyruvate

• 2 NADH

• 2 ATP

What are the 2 purposes of the glucose-6-phosphate intermediate in glycolysis?

1. Anchors in the cell

2. Sets up for further activation

At which step during glycolysis does the molecule become committed to glycolysis?

Phosphofructokinase reaction.

In the last step of the energy investment phase of glycolysis, why is DHAP converted to GAP despite ΔG being positive?

B/c G3P gets further consumed and therefore Le Chatelier’s principle occurs.

Why is triose phosphate isomerase in glycolysis considered a “perfect” enzyme?

B/c its rate of catalysis is diffusion limited- the binding step is rate-determining not the chemical step.

Which reactions of glycolysis are irreversible?

• Hexokinase

• Phosphofructokinase

• Pyruvate kinase

Why does glucose need to be transported into the brain?

b/c it lacks enzymes needed for gluconeogenesis

In gluconeogensis, what enzymes replace the following used for glycolysis?

• Hexokinase

• Phosphofructokinase

• Pyruvate kinase

• Glucose-6-phosphatase

• Fructose-1,6-bisphosphatase

• Pyruvate carboxylase & phosphoenolpyruvate carboxykinase

What is the name of the intermediate formed in gluconeogenesis but not glycolysis?

OxaloacetateWhy

Why is it important for glycolysis and gluconeogenesis to be controlled reciprocally?

To avoid futile cycles that would result in the net consumption of ATP.

What is feed-back inhibition? What is the goal of this type of regulation?

The final product in a metabolic path inhibits an enzyme earlier in the path. Prevents accumulation of products.

What is feed-forward activation? What is the goal of this type of regulation?

Early product in metabolic path activates an enzyme later in the pathway. Prevents the build-up of waste intermediates by “pressing the gas”.

How is fructose-2,6-bisphophate formed to be used in regulation?

What is the reverse rxn catalyzed by?

• Formed by fructose-6-phosphate & PFK-2

• Fructose-2,6-bisphosphatase (FBPase-2)

What is the relationship between PEP and PFK-1?

Binding of PEP deactivates PFK-1

What is special about the enzyme(s) containing PFK-2 and FBPase-2 activities?

It is a bifunctional enzyme. Phosphorylation causes conformational changes to ensure only 1 activity occurs at a time.

What is the role of Fructose-2,6-bisphosphate in the regulation of glycolysis and gluconeogenesis?

It serves as a switch. When there is lots, it drives the PFK-1 which drives glycolysis. When there is little, it drives the F1,6BPase which drives gluconeogenesis.

How are the PFK-2 and FBPase-2 activities regulated?

• Glucagon indicates low blood glucose levels

• Glucagon binds to receptor to stimulate activation of PKA through cAMP

• PKA phosphorylates PFK-2, inactivating it

• Fru-2,6-BP levels drop causing Fru-6-P levels to increase and stimulate gluconeogenesis.

What is the purpose of glycogen?

To provide a ready store of glucose.