Regulation of Glycolysis & Gluconeogenesis

Question 1

How is short-term regulation accomplished?

Answer 1

Changes in enzymatic activity

Question 2

How can the activity of an enzyme be altered? We discussed four ways in lecture.

Answer 2

1. Allostery
2. Covalent modification
3. Substrate concentration change
4. Compartmentalization

Question 3

What is allosteric regulation?

Answer 3

Regulation that occurs as a result of some small molecule that inhibits or activates an enzyme by binding somewhere besides the enzyme’s active site

Question 4

What types of covalent modifications are common in regulation?

Answer 4

Phosphorylation

Methylation

Acetylation

Question 5

What amino acid residues are most often targeted for phosphorylation?

Answer 5

Serine

Tyrosine

Question 6

How does the change in substrate concentration regulate an enzyme’s activity?

Answer 6

When [S] ≤ k_m a change in substrate concentration will affect the rate

When [S] >>> k_m changes in substrate concentration are unlikely to affect the rate as it is insensitive to substrate concentration changes

Question 7

How is long-term regulation accomplished in the cell?

Answer 7

Changing the rate of synthesis and degradation of specific enzymes in the pathway

Question 8

What do we mean by compartmentalization?

Answer 8

The enzyme and substrate are separated usually via binding to a regulatory protein

Question 9

What is v_max equal to?

Answer 9

v_max = k_cat [E]_total

Question 10

What is k_cat?

Answer 10

Catalytic constant

Question 11

If the enzyme concentration increases, what happens to v_max?

Answer 11

v_max increases

Question 12

Expression of a gene is controlled by specific sequences in the DNA that are recognized by _________________ ______________.

Answer 12

Transcription factors

Question 13

To what do transcription factors bind?

Answer 13

Response elements near the promoter

Question 14

What do transcription factors generall do?

Answer 14

Increase or decrease transcription of a particular gene

Question 15

_________________________ is also involved in long-term regulation and is associated with the stability of mRNA, translation rate, and protein turnover.

Answer 15

Degradation

Question 16

The regulation of glycolysis and gluconeogenesis occurs at the three ____________________ steps in the pathways.

Answer 16

Irreversible

Question 17

In what manner are the three irreversible steps in glycolysis and gluconeogenesis regulated?

Answer 17

A reciprocal manner

Question 18

The three irreversible steps of glycolysis and gluconeogenesis operate far from or close to equilibrium?

Answer 18

Far from

Question 19

How are reactions in metabolic pathways that operate near equilibrium regulated?

Answer 19

Changes in substrate and product concentration - Le Chatelier’s Principle

Question 20

What is the first major point of (enzymatic) regulation discussed in class?

Answer 20

Hexokinase and glucose-6-phosphatase

Question 21

Which hexokinase is distributed mainly in muscle tissue?

Answer 21

Hexokinase II

Question 22

What hexokinase is distributed in the liver?

Answer 22

Hexokinase IV or glucokinase

Question 23

Hexokinase IV is also known as ______________________ and resides in the liver.

Answer 23

Glucokinase

Question 24

There are several different forms or __________________________ of hexokinase in the human genome; these forms have different affinities for their substrates as well as different regulatory components.

Answer 24

Isozymes

Question 25

Based on the following Michaelis-Menten plot, what are the approximately k_m values for hexokinase and glucokinase?

Answer 25

Hexokinase ~ 0.25 mM Glucogkinase ~6 mM

Question 26

Hexokinase has a much lower k_m value for glucose than does glucokinase. Why?

Answer 26

A low k_m means that an enzyme has a higher affinity for a substrate; in the muscle, we want hexokinase to trap glucose in the cell for usage. This means that we want hexokinase II to have a low k_m or a high affinity for glucose. In the liver, we want to release glucose into the bloodstream and thereby maintain blood glucose concentrations. This means that we want glucokinase to have a high k_m or a low affinity for glucose.

Question 27

The v_max of hexokinase II occurs at _____ (low or high) concentrations of glucose.

Answer 27

Low

Question 28

The v_max of glucokinase occurs at _______ (low or high) concentraitons of glucose.

Answer 28

High

Question 29

Which enzyme - hexokinase II or hexokinase IV/glucokinase - is inhibited by glucokinase regulatory protein?

Answer 29

Glucokinase IV/hexokinase IV

Question 30

What are the five characteristics of hexokinase II discussed in class?

Answer 30

1. Tissue distribution - in muscle (mainly) 2. k_m - ~0.25 mM (glucose) 3. v_max - occurs at low concentrations of glucose 4. Inhibition by G6P - yes (allostery) 5. Inhibition by glucokinase regulatory protein - no

Question 31

Which enzyme - hexokinase II or hexokinase IV/glucokinase - is inhibited by glucose-6-phosphate?

Answer 31

Hexokinase II

Question 32

What is the biological significance of the differences between hexokinase II and hexokinase IV/glucokinase?

Answer 32

Muscle is a consumer of glucose; hexokinase's role is to maintain the glucose concentration in the cell so that it can be consumed for energy. Liver maintains glucose concentrations in the blood; the liver needs to send glucose out of the cell; it has a higher km so as to prevent the phosphorylation of glucose, which would trap glucose in the cell

Question 33

Glucokinase is also regulated by \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 33

Compartmentalization

Question 34

At high concentrations of fructose-6-phosphate, the regulatory protein ______________________ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ _____________ (GKRP) binds to glucokinase and sequesters it within the nucleus.

Answer 34

Glucokinase regulatory protein

Question 35

Why do we want glucokinase sequestered at high levels of fructose-6-phosphate?

Answer 35

In the liver, high levels of fructose-6-phosphate indicate that the glucose concentration has decreased outside of the cell - we need to sequester glucokinase so that any incoming glucose remains unphosphorylated and free to exit the cell and stabilize blood glucose levels

Question 36

Why do we want glucokinase active at high levels of glucose?

Answer 36

In the liver, high levels of glucose suggest that there is too much glucose in the blood, we we want to release glucokinase and phosphorylate glucose, trapping it in the cell and thus returning blood glucose levels to normal.

Question 37

What glucose transporter is present in the liver?

Answer 37

GLUT-2

Question 38

At ________________ (high or low) cellular glucose concentrations, glucokinase moves into the cytosol and becomes active. At ____________________ (high or low) cellular glucose concentrations, glucokinase regulatory protein binds glucokinase and sequesters it in the nucleus.

Answer 38

High Low

Question 39

At ______________________ (high or low) levels of fructose-6-phosphate, glucokinase is active. At ____________________ (high or low) levels of fructose-6-phosphate, glucokinase is inactive and sequestered in the nucleus.

Answer 39

Low High

Question 40

Glucose-6-phosphatase is not under allosteric control. Instead it is controlled by concentrations of \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 40

Glucose-6-phosphate

Question 41

If the concentraiton of glucose-6-phosphate increases in the liver, what happens to the activity levels of glucose-6-phosphatase?

Answer 41

It increases; we want to produce glucose to leave the cell

Question 42

When is synthesis of glucose-6-phosphatase induced?

Answer 42

When blood glucose levels drop

Question 43

Glucose-6-phosphatase is under what hormone's control?

Answer 43

Glucagon

Question 44

What is the second major point of regulation discussed in class (AKA THE MAJOR REGULATION STEP)?

Answer 44

Phosphofructokinase-1 and fructose-1,6-bisphosphatase-1

Question 45

What is the committed step of glycolysis?

Answer 45

The transformation of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase-1

Question 46

Why can't PFK-1 and FBPase-1 operate at the same time?

Answer 46

Because the sum of the two steps is ATP + H20 --\> ADP + Pi; it is energetically wasteful and futile

Question 47

What is the energy status of the cell related to?

Answer 47

Concentrations of ATP and AMP

Question 48

High ATP levels indicate that the cell is energy \_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 48

Rich

Question 49

High AMP levels indicate that the cell is energy \_\_\_\_\_\_\_\_\_\_\_.

Answer 49

Poor

Question 50

Which is a better indicator of the cell's energy status: ATP or AMP?

Answer 50

AMP

Question 51

How does ATP affect the activity of PFK-1?

Answer 51

ATP is an allosteric inhibitor of PFK-1

Question 52

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ kinetics indicate allostery.

Answer 52

Sigmoidal

Question 53

How does fructose-2,6-bisphosphate affect the activity of PFK-1?

Answer 53

F26BP is an allosteric activactor of PFK-1

Question 54

\_\_\_\_\_\_ is an allosteric inhibitor of PFK-1 while ____ is an allosteric activator of PFK-1 for its substrate fructose-6-phosphate.

Answer 54

ATP F26BP

Question 55

How do ATP and F26BP act as allosteric modulators of PFK-1?

Answer 55

They decrease or increase the affinity of PFK-1 for F6P ATP decreases affinity; F26BP increases affinity

Question 56

How does F26BP affect the activity of FBPase-1?

Answer 56

F26BP is an allosteric inhibitor of FBPase-1

Question 57

\_\_\_\_\_\_\_\_\_\_\_\_ is an allosteric inhibitor of fructose-1,6-bisphosphatase-1, lowering its affinity for the substrate, fructose-1,6-bisphosphate.

Answer 57

Fructose-2,6-bisphosphate

Question 58

What are three activators of PFK-1?

Answer 58

1. F26BP 2. AMP 3. ADP

Question 59

What are two inhibitors of PFK-1?

Answer 59

1. ATP 2. Citrate

Question 60

What are activators for FBPase-1?

Answer 60

None were discussed

Question 61

What are two inhibitors of FBPase-1?

Answer 61

1. F26BP 2. AMP

Question 62

Where does fructose-2,6-bisphosphate come from?

Answer 62

It is derived from the phosphorylation of fructose-6-phosphate by phosphofructokinase-2

Question 63

PFK2 and FBPase-2 are part of a _______________ enzyme.

Answer 63

Bifunctional

Question 64

When is the FBPase-2 activity of PFK2/FBPase2 "on"?

Answer 64

When the enzyme is phosphorylated

Question 65

When is FBPase-2 activity of PFK2/FBPase 2 "off"?

Answer 65

When the enzyme is dephosphorylated

Question 66

What enzyme catalyzes the hydrolysis of fructose-2,6-bisphosphate to fructose-6-phosphate?

Answer 66

Fructose-2,6-bisphosphatase-2

Question 67

The concentration of ________________________________ is critically important for the regulation of glucose metabolism; its concentration is regulated by hormonal signaling pathways

Answer 67

Fructose-2,6-bisphosphate

Question 68

The third major point of regulation involves pyruvate kinase versus _________________ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ and PEP carboxykinase.

Answer 68

pyruvate carboxylase

Question 69

\_\_\_\_\_\_\_\_\_\_\_\_\_ ____________ exists in different forms depending upon tissue type.

Answer 69

Pyruvate kinase

Question 70

In all tissue types, what four molecules *inhibit* pyruvate kinase?

Answer 70

1. ATP 2. Acetyl-CoA 3. Long-chain fatty acids 4. Alanine

Question 71

In the liver isoform, pyruvate kinase is also regulated by ______________________ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 71

Covalent modification

Question 72

What does phosphorylation by PKA of pyruvate kinase in the liver accomplish?

Answer 72

Inactivation of pyruvate kinase

Question 73

Pyruvate carboxylase is regulated by ATP. High levels of ATP are \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 73

Stimulatory

Question 74

PEP carboxykinase is regulated transcriptionally via the glucagon and insulin signals. What does each signal cause?

Answer 74

Glucagon increases transcription of PEP carboxykinase Insulin decreases transcription of PEP carboxykinase