Download Lecture 12: Enzymes of Metabolism: An Introduction Reference

Lecture 12: Enzymes of Metabolism: An Introduction
Reference: Lecture notes only, no reading assigned
1. As a review, list the 6 general classes of enzymes and name the two major types
of regulation for metabolic enzymes.
a. Classes of Metabolic Enzymes
i. Oxidoreductases
ii. Transferases
iii. Hydrolases
iv. Lyases
v. Isomerases
vi. Ligases
b. Regulation for Metabolic Enzymes
i. Allosteric modulators
1. Positive or activator
2. Negative or inhibitor
ii. Covalent Modification
1. Mainly phosphorylation
2. Define in simple terms the function of pyruvate dehydrogenase and pyruvate
carboxylase.
a. Pyruvate dehydrogenase and pyruvate carboxylase are the two key
enzymes that participate in the TCA cycle which occurs in the
mitochondria
b. PDH: it’s main job is to convert pyruvate to acetyl CoA so that it can enter
the TCA cycle
c. Pyruvate carboxylase: it converts pyruvate to oxaloacetate (OAA). When
acetyl CoA levels are high, cell need OAA to keep the TCA cycle running
3. List the cofactors required for PDH activity.
a. Thiamin pyrophosphate (Vitamin B1 derivative): involved in group
transfers. Helps catalyze the decarboxylation reaction
b. Lipoate: Energy Reactions. Lipoate hold the acetyl group and transfers it
to acetyl Coa, which carry the acetyl group into the TCA cycle.
c. FAD (riboflavin derivative): Energy reactions. Helps to regenerate lipoate
for the next reaction.
4. Define in general terms the mechanism(s) of regulation of pyruvate
dehydrogenase and pyruvate carboxylase (i.e., allosteric? phosphorylation?).
a. Pyruvate dehydrogenase is regulated by covalent phosphorylation (it’s
inactive when it is phosphorylated)
i. Positive modulators/activators: NAD+, CoA
ii. Negative modulators/inhibitors: NADH, acetyl CoA
b. Pyruvate Carboxylase is activated by acetyl CoA
i. High levels of acetyl CoA activate pyruvate carboxylase to convert
pyruvate to OAA (cell signal)
5. Define in general terms the function of carnitine:palmitoyl transferase I and II
and acetyl CoA carboxylase. Again, no real detail is needed since these
pathways have not been covered.
a. CPT I enzyme: exhanges carnitine for CoA bound to Fatty Acid (FA). Only
carnitine FA can be transported in the mitochondrial matrix. It allows FA to
be transported into the mitochondrial matrix to be oxidized. Once inside,
FA is converted back into FA-CoA (the active form).
b. Acetyl CoA carboxylase: is used to transform glucose into fats like
palmitate and palmitoyl CoA
6. Explain why newly synthesized fatty acids are not immediately transported into
mitochondria for oxidation.
a. FA must first be bound to carnitine in order to enter the mitochondrial
matrix to be oxidized
b. FA are not immediately transported into the mito in order to prevent a
“futile cycle” where energy is spent to make a molecule that is
degraded right away.
7. Explain the term “committed step” for a metabolic pathway.
a. A committed step is an irreversible enzymatic reaction
b. In Fatty Acid synthesis, the enzyme acetyl CoA carboxylase is the
committed step. It carboxylates acetyl CoA and changes ATP to ADP to
form Malonyl CoA
8. Define in simple terms the goal of glycolysis and role of PFK-1 in this pathway.
a. Glycolysis is the pathway for the anaerobic oxidation of glucose to
pyruvate
b. Glucose enters cell and gets phosphorylated trapping it in the cell.
c. Glucose-6-P is isomerized to fructose-6-P (reversible)
d. Conversion of fructose-6-P to fructose-2,6-P by allosteric regulation
by PFK-1 enzyme (committed step)
e. F-2,6-P must turn into pyruvate
f. PRK-1 is required for glycolysis, deficiencies result in little or no
mitochondria—muscle cramps after exercise
9. Explain the significance of HMG-CoA reductase and mevalonate in the
cholesterol synthesis pathway. Include in your answer why HMG-CoA reductase
catalyzes the committed step in the pathway.
a. HMG-CoA reductase: reduces HMG-CoA to Mevalonate for
cholesterol synthesis. It is a committed step
b. HMG-CoA reductase catalyzes the committed step and controls it
because while there are many uses for acetyl CoA and HMG-CoA, there is
only one use for mevalonate (cholesterol) so its synthesis must be very
controlled.
10. List and explain in very general terms the four regulation strategies for HMG-CoA
reductase.
i. Controlled by sterols- If sterols are high, HMG-CoA is degraded to
stop production of mevalonate
ii. Hormonal control causes reversible covalent phosphorylation of
HMG-CoA
iii. Activity depends on the availability of substrate, HMG-CoA
iv. Statins are competitive inhibitors of HMG-CoA reductase
1. Bind with affinity to the HMG-CoA reducatase active site and
block the active site so HMG-CoA cant bind for reaction
11. State in general terms the relationship between LDL particles, LDL receptors on
cells and cholesterol production by cells.
a. LDL particles carry cholesterol to the peripheral tissues
b. Cells contain LDL receptors that bind the LDL particles and bring them
into the cell, where they dump their cholesterol cargo.
c. Since HMG-CoA reductase is sensitive to the level of sterols (cholesterol)
in cells, this will down regulate HMG-CoA reductase.
d. This activates storage of excess cholesterol. It is reesterified to form
cholesterol ester droplets in the cytosol.
12. Describe in general terms the function of the enzymes ALT and AST, and explain
why they are considered markers of liver disease/damage.
a. ALT: alanine aminotransferase—transfers an amino group from alanine
to alpha ketoglutarate to generate glutamate, which carries the nitrogen
to be disposed
b. AST: aspartate aminotransferase—transfers an amino group from
oxaloacetate to glutamate to generate alphaketolutarate
c. Liver cells contain a much greater concentration of ALT and AST than
most other cells. The appearance of ALT or AST in blood is a sign of
cellular damage, and in particular, liver damage.
i. Hepatitis
ii. Chronic alcoholism
13. Describe in general terms the structure and function of creatine kinase, and
explain why it is considered a marker of cardiac muscle disease / damage.
a. Creatine kinase structure: consists of two subunits, B and M
i. 3 isozymes of CK:
1. BB (brain)
2. BM (heart only)
3. MM (skeletal and cardiac)
b. Creatine kinase function: catalyzes the reaction of creatine into creatine
phosphate and uses ATP into ADP
c. It is considered a marker of cardiac muscle disease/damage because
there is usually a little CK in the blood due to normal cell turnover.
However, when cells are damaged (heart attack or heart disease), there is
more than the normal amount of CK in the blood.
i. The MM isozyme is found in the heart, and so an elevated level of
CK-BM and CK-MM in the blood is indicative of heart cell damage.