Metabolism 1 and 2

Question 1

Why don’t skeletal muscles export glucose?

Answer 1

They don’t have Glucose-6-Phosphatase so can’t convert glucose-6-P –> Glucose. During states of high glucose (resting), they convert to glycogen to store

Question 2

What enzymes convert Glucose –> G-6-P? (Glucose metabolism)

Answer 2

Hexokinase (Skeletal muscle)

Glucokinase (Hepatocytes)

Question 3

What is caused by deficiency in glycolytic enzymes?

Answer 3

Hemolytic Anemia (RBCs burst, lose Hb)

Question 4

What reaction do HK/GK catalyze? What is important about the product?

Answer 4

Glucose –> G-6-P
G-6-P is more polar and impermeable so cannot leave the cell
This rxn is irreversible phosphorylation and ATP used with Mg as a cofactor

Question 5

Where is Hexokinase located? Glucokinase?

Answer 5

HK: all cell types
GK: liver and pancreas

Question 6

What is the regulator of Hexokinase?

Answer 6

Allosteric inhibition by G-6-P (feedback inhibition)

Question 7

What are the regulators of Glucokinase?

Answer 7

1. Fructose-6-P promotes translocation to nucleus (decreases activity)
2. Glucose promotes translocation to cytosol (increases activity)
3. Insulin promotes enzyme synthesis (inducible enzyme)

Question 8

List differences between Hexokinase and Glucokinase

Answer 8

Hexokinase:
- All cell types
- Constitutive enzyme (present in constant amount independent of activation)
- Low Km for glucose (saturates at low conc.), High affinity
Glucokinase:
- Liver and pancreas
- Inducible enzyme (insulin)
- High Km for glucose (not saturated at normal conc.), Low affinity

Question 9

What reaction is catalyzed by PFK-1?

Answer 9

F-6-P –> F-1,6-bisP

Question 10

What is the main regulator of PFK-1? What are the conditions that increase or decrease the regulator’s concentration?

Answer 10

Fructose-2,6-bisP is the major physiological activator of hepatic PFK-1

• High F-2,6,-bisP when insulin and blood glucose high
• Low F-2,6,bisP when glucagon and epi high (Glucagon/Epi –> cAMP –> active PKA –> phosphorylates kinase domain of PFK-2 –> F-2,6-bisP not formed)

Question 11

What is the reaction catalyzed by PFK-2?

Answer 11

Fructose-6-P –> Fructose-2,6-bisP

Question 12

What are the 2 isoforms of PFK-2 and how are they influenced?

Answer 12

1. In liver, glucagon/epi –> inhibit/phosphorylate PFK-2 kinase domain, promote phosphatase –> block F-2,6,bisP –> block glycolysis
2. In heart/skeletal muscle, Epi only –> inhibit/phosphorylate PFK-2 phosphatase domain, promote kinase –> made F-2,6-bisP –> promote glycolysis

Question 13

What reaction is catalyzed by Pyruvate Kinase?

Answer 13

PEP (phosphoenolpyruvate) –> Pyruvate

Question 14

How is Pyruvate Kinase regulated (2 activators 2 inhibitors)

Answer 14

Activate PK:

• Fructose-1,6-bisP
• Insulin: promotes dephosphorylation of PK (active)

Inhibit PK:

• ATP and Alanine
• Glucagon/Epi: promote phosphorylation (via PKA) of PK (inactive)

Question 15

What are the 3 main enzymes of glycolysis?

Answer 15

1. Hexokinase/Glucokinase
2. PFK-1
3. Pyruvate Kinase

Question 16

What 2 enzymes do glucagon/epinepherine modify in hepatic glycolysis?

Answer 16

1. Indirectly inhibits PFK-1 by phosphorylating kinase domain of PFK-2 (less F-2,6-bisP)
2. Directly phosphorylating Pyruvate Kinase

Question 17

What are the 4 fates of Pyruvate post-glycolysis?

Answer 17

1. Transamination –> Alanine
2. Pyruvate Carboxylase (caboxylation) –> Oxaloacetate
3. LDH (reduction) –> Lactate –> Cori Cycle
4. PDH (oxidation)–> Acetyl-CoA –> TCA –> ETC

Question 18

What happens with Pyruvate Carboxylase deficiency?

Answer 18

(Converts pyruvate –> oxaloacetate)

• Will cause increased alanine, lactate, pyruvate concentrations
• Symptoms: developmental delay, recurrent seizures, metabolic acidosis

Question 19

What happens with PDH deficiency?

Answer 19

(allows pyruvate to enter TCA cycle)

• Will cause increased pyruvate and lactate concentrations
• Symptoms: micocephaly, poor muscle coordination, mental retardation

Question 20

What are the 3 ways NAD+ can be regenerated in the cytoplasm?

Answer 20

1. LDH
2. Malate-Aspartate Shuttle
3. Glycerol-Phosphate Shuttle

Question 21

What reaction does the PHD complex catalyze?

Answer 21

Pyruvate + Coenzyme A (CoASH) + NAD+ –> Acetyl-CoA + NADH

Question 22

What vitamins/cofactors are needed for PDH?

Answer 22

Vitamin B1 (Thiamine-TPP)
Vitamin B2 (Riboflavin-FAD)
Vitamin B3 (Niacin-NAD)
(and Vitamin B5)

Question 23

What occurs during LDH-A?

Answer 23

LDHA is lactate dehydrogenase A deficiency

• Patient can’t maintain exercise because can’t do glycolysis to make ATP needed for muscle contraction anaerobically
• NAD+ becomes limiting during exercise and flux through glyceraldehyde-3-P-dehydrogenase reaction inhibited

Question 24

What regulates PDH?

Answer 24

Acetyl-CoA and NADH:

1. allosterically inhibit PDH
2. Promote phosphorylation/inhibition of PDH

Question 25

What 2 enzymes metabolize galactose?

Answer 25

1. Galactokinase

2. Galactose-1-P-uridyltransferase

Question 26

What occurs in Galactosemia? Which enzyme deficient?

Answer 26

Most commonly Galactose-1-P-uridyltransferase

• causes accumulation of galactose
• causes cataracts (among other problems)

Question 27

How is Hereditary Fructose Intolerance caused? What is effect?

Answer 27

Aldolase B deficiency

• Fructose-1-P trapping of Pi –> cannot make ATP –> ATP levels fall quickly
• Hypoglycemia, Vomiting, Jaundice, Hepatic failure