Exam 3: Glycolysis

Question 1

Know that red blood cells, skeletal muscle cells, and brain cells are highly or exclusively dependent on glucose/glucose 6-phosphate as an energy source. And know why.

Answer 1

1. RBCs : they lack mitochondria and therefore can only undergo glycolysis which requires glucose
2. Brain: only glucose can cross the blood brain barrier.
3. Skeletal muscle: during exercise anaerobic glycolysis occurs which produces ATP faster

Question 2

Define Glycolysis

Answer 2

• A sequence of reactions that metabolizes one molecule of of glucose to 2 molecules of pyruvate and generates the net production of two molecules of ATP
• This is an anaerobic process
• Pyruvate can be completely oxidized under aerobic conditions generating more ATP

Question 3

Which has a lower affinity for glucose, glucokinase or hexokinase?

Answer 3

glucokinase, found in the liver

Question 4

When will the liver use glucose to produce ATP, instead of glucose storage?

Answer 4

When there is a high level of glucose, such as after a meal.

Question 5

What is the reaction of glycolysis that reduces NAD+ to NADH?

Answer 5

Step 6: The coversion of Glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate by glyceraldehyde 3-phosphate dehydrogenase (oxidative-reduction rxn) (twice per each glucose molecule)

Question 6

What is the significance of the oxidation-reducation rxn where NAD+ is reduced to NADH?

Answer 6

There is only a limited amount of NAD+ in the cell, the NADH formed by this reaction must be reoxidized to NAD+ for glycolysis to continue.

Question 7

What are two mechanisms for reoxidizing NADH?

Answer 7

1. NADH-linked conversion of pyruvate to lactate.
2. Oxidation of NADH via the respiratory chain (requires shuttles)

Question 8

What is the importance of the reduction of pyruvate to lactate in anaerobic glycolysis?

Answer 8

The formation of lactate is the major fate of pyruvate in lens and cornea of the eye, kidney medulla, testes, leukocytes and RBCs, b/c these are poorly vascularized and/or lack mitochondria for aerobic respiration

Also, in exercising skeletal muscle NADH production exceeds the oxidative capacity of the respiratory chain. This results in an elevated NADH/NAD+ ratio, favoring reduction of pyruvate to lactate. Therefore, during intense exercise, lactate accumulated in muscle, causing a drop in the intracellular pH, potentially resulting in cramps. Much of the lactate diffuses into the bloodstream and can be used by the liver to make glucose.

Question 9

What are the three irreversible reaction of glycolysis?

Answer 9

1. Glucose to Glucose 6-Phosphate via HEXOKINASE
2. F6P to F1,6,BP via PFK1
3. Phosphoenolpyruvate to Pyruvate via Pyruvate Kinase

Question 10

What is considered the “committed step” of Glycolysis?

Answer 10

The conversion of F6P to F 1,6BP via phosphofructose kinase-1 (PFK1). Before this happened, glucose could go to the pentose phosphate pathway.

Question 11

Where is glucokinase found?

Answer 11

in liver parenchymal cells and Beta-cells fo the pancrease.

Question 12

What controls the regulation of PFK1 in muscle cells?

Answer 12

PFK-1 is inhibited allosterically by elevated levels of ATP, which act as an “energy-rich” signal inidicating an abundance of high energy compounds. Conversely, PFK-1 is activated allosterically by high concentractions of AMP, which signal that the cell’s energy stores are depleted.

Question 13

What controls the regulation of PFK1 in liver cells?

Answer 13

• Fructose 2,6-bisposphate is the most potent activator of PFK1, it can activate the enzyme even with high ATP levels.
• F-2,6-BP is formed by PFK2.
• PFK2 can function as a kinase to produce F-2,6-BP and as a phosphatase that dephosphorylates F-2,6-BP back to F6P
• Inhibited by citrate

Question 14

How is pyruvate kinase regulated in the Liver?

Answer 14

By allosteric effectors and covalent modification.

Question 15

What reactions of Glycolysis require ATP?

Answer 15

• Step 1: Glucose to glucose 6-phosphate
• Step 3: F6P to F-1,6-BP

Question 16

What steps of glycolysis produce ATP?

Answer 16

• step 7: 1,3BPG to 3-PG
• Step 10: PEP to pyruvate

Question 17

How does anaerobic glycolysis yeild a net of 2 ATPs for every glucose molecule oxidized?

Answer 17

In step 4, Aldolase cleaves F-1,6-BP to DHAP and G3P (2 3-carbon phosphate compounds), which is reversible and not regulated.

Question 18

Sucrose is a disaccharide of _______ and ________.

Answer 18

Glucose and Fructose

Question 19

Lactose is a disaccharide of ___________ and ___________.

Answer 19

glucose and galactose

Question 20

How does fructose enter the glycolytic pathway?

Answer 20

it is converted into a glycolytic intermediate through fructokinase

Question 21

How does galactose enter in to glycolysis?

Answer 21

through glycolytic intermediates with galactokinase.

Question 22

How can the glucose from glucose-6-phosphate “escape” the liver into the bloodstream but not from most other cells?

Answer 22

Hydrolysis of glucose-6-phosphate by glucose-6phosphatase bypasses the irreversible hexokinase reaction, and provides an energetically favorable pathway for th eformation of free glucose.

Question 23

Define Insulin:

Answer 23

A polypeptide hormone produced by the Beta-cells of the islets of langerhans. Insulin is the most important hormone coordinating the use of fuels by tissues. It is closey related with the release of glucagon by pancreastic alpha-cells.

Question 24

Define Glucagon:

Answer 24

Glucagon is secreted by the alpha cells of the pancreatic islets of langerhans. It acts to maintain bloog glucose levels by activating hepatic glycogenolysis and gluconeogenesis. Works to oppose many of the actions of insulin.