Enzymes of Metabolism

Question 1

Lactate Dehydrogenase

Answer 1

Lactate + NAD –> pyruvate + NADH

Question 2

Pyruvate Dehydrogenase

Answer 2

Pyruvate –> acetyl coA

Allows pyruvate to enter TCA cycle

Question 3

Hexokinase

Answer 3

Glucose –> glucose-6-phosphate

Found in non-liver cells
Requires ATP and is stimulated by insulin

Question 4

Glucokinase

Answer 4

Glucose –> glucose-6-phospate

Found in the liver
Requires ATP and is stimulated by insulin

Question 5

Phosphofructokinase 1

Answer 5

Enzyme of the committed step of glycolysis
Begins the series of reactions that results in acetyl coA –> 2 pyruvate, 2 ATP, 2 NADH (OR lactate + NAD+ in anaerobic metabolism)

Stimulated by high [AMP] and by insulin.
Inhibited by high [ATP], high [citrate], and by glucagon.

Question 6

Pyruvate Carboxylase

Answer 6

Pyruvate –> oxaloacetate (creation of oxaloacetate is anaplerotic! Adds to TCA)

Question 7

Glutamate hydrogenase

Answer 7

Glutamate + pyruvate –> a-ketoglutarate + alanine

Requires B6
a-ketoglutarate is a TCA intermediate, so a reaction producing it is anaplerotic.

Question 8

Phosphoenolpyruvate carboxylase

Answer 8

Oxaloacetate –> glyceraldehyde 3 ( DHAP).

This is an important enzyme in gluconeogenesis and is also involved in generating glycerol from pyruvate (glycerol needed for TAG synthesis)
Also called PEPCK

Question 9

Fructose-1-6-biphosphatase

Answer 9

F-1-6-BiP –> F-6-P

Important step in gluconeogenesis

Question 10

Glucose-6-Phosphatase

Answer 10

G-6-P –> glucose

Important step in gluconeogenesis
Exists in liver cells (NOT skeletal muscle)
Without removing the phosphate group, glucose would not be able to travel outside of the cell it was made in

Question 11

Phosphoglucomutase

Answer 11

G-6-P –> G-1-P

Important step in glycogen synthesis and glycogenolysis
Moves the phosphate group

Question 12

Branching enzyme

Answer 12

Adds branches via a-1-6 bonds during glycogen synthesis

Question 13

Glycogen synthase

Answer 13

Adds more glucose monomers via a-1-4 bonds during glycogen synthesis

Question 14

Debranching enzyme

Answer 14

Cleaves a-1-6 bonds in glycogen, breaking branches
Stimulated by glucagon and epinephrine
Important step in glycogenolysis

Question 15

Glycogen phosphorylase

Answer 15

Cleaves a-1-4 bonds in glycogen, pulling apart glucose monomers –> G-1-P
Important step in glycogenolysis

Question 16

Glutathione reductase

Answer 16

Facilitates the e- donation from NADPH to glutathione

Important enzyme in ROS neutralization

Question 17

Glutathione peroxidase

Answer 17

Hydrogen peroxide + glutathione –> H20
Facilitates the e- donation from glutathione to hydrogen peroxide
Important enzyme in ROS neutralization

Question 18

Hormone-sensitive-lipase

Answer 18

Breaks down TAGs into 3 fatty acids + glycerol

Stimulated by glucagon, epinephrine, or ACTH
Inhibited by insulin
Important step in fatty acid oxidation

Question 19

Carnitine-palmitoyl transferase I

Answer 19

CPT I
Fatty acyl CoA + carnitine –> fatty acyl carnitine + CoA

Important step in fatty acid oxidation because fatty acyl coA needs a 2-phase “carnitine shuttle” to get into mitochondrial matrix

• Inhibited by malonyl CoA (produced during fatty acid synthesis) and insulin
• Stimulated by AMP-PK

Question 20

Carnitine-palmitoyl transferase II

Answer 20

CPT II
Fatty acyl carnitine + CoA –> fatty acyl coA + carnitine

Important step in fatty acid oxidation because fatty acyl coA needs a 2-phase “carnitine shuttle” to get into mitochondrial matrix

Carnitine travels back to outer mitochondrial membrane to be recycled by the shuttle

Question 21

Dynamics of malonyl coA regulation

Answer 21

Malonyl coA is an important regulator of fatty acid synthesis and oxidation

It is created as an intermediate during fatty acid synthesis
High levels of malonyl coA will inhibit CPT I and thus inhibit fatty acid oxidation

It is produced from acetyl CoA —> malonyl coA via the enzyme acetyl CoA carboxylase

Insulin stimulates acetyl coA carboxylase activity via dephosphorylation by PP2, increases malonyl coA, and stimulates fatty acid synthesis (fed = store fat)

Glucagon inhibits acetyl coA carboxylase activity via phosphorylation by AMP-PK, reduces malonyl coA, and stimulates fatty acid oxidation (fasted = burn fat)

Question 22

Fatty acid synthase

Answer 22

Malonyl coA –> palmitate (a 16-c fatty acid)

Requires 8 acetyl coA and 14 NADPH to create palmitate
Complex enzyme that catalyzes a number of reactions important in fatty acid synthesis.

Question 23

PP2

Answer 23

Removes phosphate from acetyl coA carboxylase, which activates it and allows fatty acid synthesis to run.

Stimulated by insulin.

Facilitates synthesis of malonyl coA and therefore inhibits fatty acid oxidation.

Question 24

AMP-PK

Answer 24

Phosphorylates acetyl coA carboxylase, which deactivates it and inhibits fatty acid synthesis.

Stimulated by glucagon.

Inhibits synthesis of malonyl coA and therefore favors fatty acid oxidation.

Question 25

Acyl CoA dehydrogenase

Answer 25

Fatty acyl coA –> enoyl coA

Exists in one of 4 isozyme forms:
VLCAD, LCAD, MCAD, SCAD
and has specificity for length of fatty acid it acts upon.

Important step in fatty acid oxidation.

Question 26

Thiolase

Answer 26

Acetoacetate –> acetyl coA

Important step in fatty acid oxidation.

Occurs in last round of B-oxidation and runs backwards when [acetyl coA] is high.

Acetoacetate is a ketone body and enters circulation during extended fasting states, being converted back into acetyl coA at their target cells.

Question 27

Aminotransferase

Answer 27

Reacts an amino acid + alpha ketoacid to alpha ketoglutarate and glutamate

Also called transaminase
Requires pyridoxal phosphate - a B6 derivative
Important step in amino acid break down

Question 28

Aspartate aminotransferase

Answer 28

Serum levels often elevated in liver disease

Question 29

Alanine aminotransferase

Answer 29

Facilitates alpha ketoglutarate + glatamate –> alanine + pyruvate

Serum levels often elevated in liver disease

Question 30

Alkaline phosphatase

Answer 30

Serum levels often elevated in liver disease