Biochem - Metabolism (Basics on pathways)

Question 1

What are 5 enzymes in glycolysis/gluconeogenesis that catalyze irreversible reactions that serve as important points of regulation? What reaction does each enzyme catalyze?

Answer 1

(1) Hexokinase/Glucokinase: Glucose => Glucose-6-Phosphate (2) Glucose-6-Phosphatase: Glucose 6-Phosphate => Glucose (3) Phosphofructokinase-1: Fructose-6-phosphate => Fructose-1,6-bisphosphate (4) Fructose-1,6-bisphosphatase: Fructose-1,6-bisphosphate => Fructose-6-phosphate (5) Pyruvate kinase: Phosphoenolpyruvate (PEP) => Pyruvate

Question 2

What are 4 enzymes in the TCA cycle that catalyze irreversible reactions that serve as important points of regulation? What reaction does each enzyme catalyze?

Answer 2

(1) Pyruvate dehydrogenase: Pyruvate => Acetyl-CoA (2) Citrate synthase: Oxaloacetate (with addition of Acetyl-CoA) => Citrate (Note: bridge between glycolysis and TCA cycle) (3) Isocitrate dehydrogenase: Isocitrate => alpha-ketoglutarate (4) alpha-ketoglutarate dehydrogenase: alpha-ketoglutarate => Succinyl-CoA

Question 3

What are 2 enzymes connecting metabolites of glycolysis and the TCA cycle that catalyze irreversible reactions that serve as important points of regulation? What reactions does each enzyme catalyze?

Answer 3

(1) Pyruvate carboxylase: Pyruvate => Oxaloacetate (2) PEP carboxykinase: Oxaloacetate => Phosphoenolpyruvate (PEP)

Question 4

What is the defective enzyme in mild galactosemia? What reaction does it catalyze? To what overall process does this reaction belong and/or relate, and how?

Answer 4

Galactokinase; Galactose => Galactose-1-phosphate; Digestion of galactose (into galactose-1-phosphate => glucose-1-phosphate => glucose-6-phosphate) for glycolysis

Question 5

What is the defective enzyme in severe galactosemia? What reaction does it catalyze? To what overall process does this reaction belong and/or relate, and how?

Answer 5

Galactose-1-phosphate uridyltransferase; Galactose-1-phosphate => Glucose-1-phosphate; Digestion of galactose (from galactose => galactose-1-phosphate and into glucose-1-phosphate => glucose-6-phosphate) for glycolysis

Question 6

What is the defective enzyme in von Gierke’s disease? What reaction does it catalyze? To what overall process does this reaction belong and/or relate?

Answer 6

Glucose-6-phosphatase; Glucose-6-phosphate => Glucose; Gluconeogenesis (reversal of glycolysis, or production of free glucose)

Question 7

What is the defective enzyme in G6PD deficiency (fully written out)? What reaction does it catalyze? To what overall process(es) does this reaction belong and/or relate, and how?

Answer 7

Glucose-6-phosphate dehydrogenase; Glucose-6-phosphate => 6-phosphogluconolactone; Connects glycolysis to HMP shunt

Question 8

What reaction does Transketolase catalyze? To what overall process(es) does this reaction belong and/or relate, and how?

Answer 8

Transketolase: Ribulose-5-phosphate =»> Fructose-6-phosphate; Connects HMP shunt back to glycolysis

Question 9

What is the defective enzyme in essential fructosuria? What reaction does it catalyze? To what overall process does this reaction belong and/or relate?

Answer 9

Fructokinase; Fructose => F1P; Digestion of Fructose (through F1P into[ DHAP or Glyceraldehyde] => Glyceraldehyde 3-P) into glycolysis

Question 10

What is the defective enzyme in fructose intolerance? What reaction does it catalyze? To what overall process does this reaction belong and/or relate?

Answer 10

Aldolase B; F1P => DHAP OR Glyceraldehyde; Digestion of Fructose (DHAP OR Glyceraldehyde => Glyceraldehyde-3P) into Glycolysis

Question 11

Where is Aldolase A versus Aldolase B found? Again, what reaction does Aldolase catalyze, and in what process is it involved?

Answer 11

Aldolase B (liver), A (muscle); F1P => DHAP OR Glyceraldehyde; Digestion of Fructose (DHAP OR Glyceraldehyde => Glyceraldehyde-3P) into Glycolysis

Question 12

What reaction does HMG-CoA Reductase catalyze? To what overall process(es) does this reaction belong and/or relate?

Answer 12

HMG-CoA reductase: HMG-CoA => Mevalonate; From Acetyl-CoA (Acetyl-CoA => Acetoacetyl-CoA => HMG-CoA) to Cholesterol synthesis (Mevalonate =»> Cholesterol)

Question 13

What reaction does Ornithine transcarbamylase catalyze? To what overall process(es) does this reaction belong and/or relate?

Answer 13

Ornithine transcarbamylase: Ornithine (with addition of Carbomyl phosphate from NH3 + CO2) => Citrulline; Urea cycle

Question 14

What are 3 major metabolic enzymes that require thiamine cofactor (TPP)? Include the reactions catalyzed by each, and the overall pathways to which they contribute.

Answer 14

(1) Transketolase: Ribulose-5-phosphate => Fructose-6-Phosphate (connecting HMP shunt back to glycolysis) (2) Pyruvate dehydrogenase: Pyruvate => Acetyl-CoA (bridge between glycolysis and TCA cycle) (3) alpha-ketoglutarate dehydrogenase: alpha-ketoglutarate => succinyl-CoA (TCA cycle)

Question 15

What are 3 major metabolic enzymes that require biotin cofactor? Include the reactions catalyzed by each, and the overall pathways to which they contribute.

Answer 15

(1) Pyruvate carboxylase: Pyruvate => Oxaloacetate (side connection between glycolysis and TCA cycle) (2) Acetyl Co-A carboxylase: Acetyl-CoA => Malonyl-CoA (connects TCA cycle to Fatty acid synthesis) (3) Propionyl-CoA carboxylase: propionyl-CoA => methylmalonyl-CoA (feeding odd-chain fatty acids, branched chain amino acids, methionine, and threonine into TCA cycle, since methylmalonyl-CoA => succinyl-CoA)

Question 16

What is a major metabolic step that requires vitamin B12 as a cofactor? Include the reaction catalyzed by this enzyme, and the overall pathway(s) to which it contributes.

Answer 16

Methylmalonyl-CoA mutase: Methylmalonyl-CoA => Succinyl-CoA; Feeding odd-chain fatty acids, branched chain amino acids, methionine, and threonine into TCA cycle

Question 17

How much ATP does aerobic metabolism of glucose produce, via what pathways/shuttles, and in what parts of the body?

Answer 17

Aerobic metabolism of glucose produces 32 net ATP via malate-aspartate shuttle (heart and liver), 30 net ATP via glycerol-3-phosphate shuttle (muscle).

Question 18

What effect does Arsenic have on glucose metabolism?

Answer 18

Arsenic causes glycolysis to produce zero net ATP

Question 19

How much ATP does anaerobic glycolysis produce?

Answer 19

Anaerobic glycolysis produces only 2 net ATP per glucose molecule.

Question 20

What metabolic advantage does ATP hydrolysis confer?

Answer 20

ATP hydrolysis can be coupled to energetically unfavorable reactions

Question 21

For each of the following carrier molecules, give what is carried in their activated form: (1) ATP (2) NADH, NADPH, FADH2 (3) CoA, lipoamide (4) Biotin (5) Tetrahydrofolates (6) SAM (7) TPP.

Answer 21

(1) Phosphoryl groups (2) Electrons (3) Acyl groups (4) CO2 (5) 1-carbon units (6) CH3 groups (7) Aldehydes

Question 22

What are 2 types of universal electron receptors? Give at least one example of each.

Answer 22

Nicotinamides (NAD+ from vitamin B3, NADP+) and Flavin nucleotides (FAD+ from vitamin B2).

Question 23

In what kind of processes is NAD+ generally used, and how?

Answer 23

NAD+ is generally used in catabolic processes to carry reducing equivalents away as NADH

Question 24

In what kind of processes is NADPH used, and how? Give 2 examples of such processes.

Answer 24

NADPH is used in anabolic processes (steroid and fatty acid synthesis) as a supply of reducing equivalents

Question 25

Of what process is NADPH a product?

Answer 25

NADPH is a product of the HMP shunt

Question 26

What are the origins of NAD+ and FAD+?

Answer 26

NAD+ from vitamin B3; FAD+ from vitamin B2

Question 27

What are 4 physiological uses for NADPH?

Answer 27

NADPH is used in: (1) Anabolic processes (2) Respiratory burst (3) Cytochrome P-450 system (4) Glutathione reductase