Enzyme Regulation

Question 1

What are some mechanisms by which inactive proteins become active proteins and vice versa?

Answer 1

By proteolysis, allosteric changes including G proteins (Gs, Gi, Gq), via second messengers (cAMP, cGMP, Ca2+), by phosphorylation

Question 2

How does the precursor angiotensingon get converted to ang II? What enzymes are
involved and what is the significance of angII in the body?

Answer 2

Renin (a protease from the kidney) cleavesangiotensinogen to angI which is then
converted to angII via the action of the enzyme ACE (another protease, endothelial cells).
Angiotensinogen –> angI –> angII.

Angiotensin II is a potent vasoconstrictor and
increases blood pressure.

Question 3

When might the RAAAS reactions take place? Under what conditions?

Answer 3

The above pathway is called RAAAS – renin, angiotensin, aldosterone, ADH. It takes
place during emergency/fight/flight, low blood pressure (hypovolemia), low sodium (hyponatremia), hypoperfusion of kidneys, extended starvation.

Question 4

What are the actions of fibrin and plasmin?

Answer 4

Fibrin promotes clotting, plasmin dissolves clots

Question 5

What is the mechanism of G proteins?

Answer 5

In the inactive form of a G protein, the alpha-subunit is bound to GDP. Binding of
ligand causes a conformational change in the receptor, triggering replacement of the GDP
with GTP. The GTP bound form of the alpha-subunit dissociates from the other two
subunits and moves to adenylyl cyclase, which becomes activated. The actions of the
alpha-ATP complex are short-lived because it has inherent GTPase activity, resulting in
rapid hydrolysis of GTP.

Question 6

Compare/contrast Gs, Gi, Gq

Answer 6

Gs = stimulatory G protein that activates adenylyl cyclase and thus increases cAMP.
Gi = inhibitory G protein that inactivates adenylyl cyclase and thus decreases cAMP.
Gq=queer/strange and it activates phospholipase C and thus increases IP3 and Ca2+

Question 7

What three enzymes are regulated by 2nd messengers? Which 2nd messengers regulate them?

Answer 7

Protein kinase A (PKA): cGMA
protein kinase G (PKG): cGMP
calcium-calmodulin kinase: Ca2+

Question 8

What are the most common 2nd messengers?

Answer 8

cAMP, cGMP, Ca2+, IP3, DAG (diacylglycerol)

Question 9

What does glycogen phosphorylase (GP) do? What activates it and inhibits it?

Answer 9

Uses phosphoric acid to break glycogen into glucose. Activated by AMP/ADP, glucagon, epinephrine, calcium kinase and inhibited by ATP, insulin, and glucose-6-P.

Question 10

What activities increase glycogen phosphorylase activity?

Answer 10

Dreaming, exercise

Question 11

Which organ prefers fatty acids and which prefers glucose?

Answer 11

Heart - fatty acids, brain - glucose

Question 12

What should you give a patient if they have overdosed on beta blockers?

Answer 12

Glucagon - Beta receptors on adipocyte normally receive norepi/epi signal to release hormone-sensitive lipase and release free fatty acids into blood, but glucagon does not need a beta receptor

Question 13

How does cAMP exert its effects on PKA?

Answer 13

Cyclic AMP (cAMP) activates protein kinase A by binding to its two regulatory
subunits, causing the release of active catalytic subunits. The active subunits catalyze the
transfer of phosphate from ATP to specific serine or threonine residues of protein
substrates. The phosphorylated proteins may act directly on the cell’s ion channels, or, if
enzymes, may become activated or inhibited.

Question 14

What are isoenzymes? Give two examples

Answer 14

Isoenzymes have the same function but different primary amino acid sequences. For
example—hexokinase and glucokinase are isoenzymes in that both add phosphates to
sugars. The numerous GLUT transports are also examples of isoenzymes.

Question 15

What are SGLTs?

Answer 15

Glucose transporter that uses secondary active transport

Question 16

What are GLUTs?

Answer 16

Glucose transporter that is facilitated transport and does not require energy

Question 17

Where are GLUT 2 transporters found? What’s their Km? What does this indicate?

Answer 17

Liver, beta cells of the pancreas.

High Km of 15-20 mM.

Hence, the rate of glucose transport is proportional to blood glucose levels.
Pancreatic Beta cells can sense glucose and adjust insulin secretion accordingly. High Km of GLUT2 in liver also assures that glucose rapidly enters liver cells only in times of plenty. When blood glucose level is low, glucose preferentially enters brain, RBCs, and other tissues because their glucose transporters have a lower Km than that of liver.

Question 18

Where are GLUT 1 and GLUT 3 transporters found? What’s their Km? What does this indicate?

Answer 18

Most tissues—namely RBCs, brain, placenta, neurons, testes.

Km ~1 mM, significantly lower than blood glucose conc. of 4-8 mM. Present in nearly all tissues

Is responsible for basal glucose uptake. Hence, GLUT1 & 3 continually transport glucose at an essentially constant rate

Question 19

Where are a GLUT 4 transporters found?

Answer 19

Km of 5mM. Found predominantly in adipose, skeletal muscle, heart.
Insulin sensitive.

Question 20

Where is SGLT-1 found? What ion does it depend on? What molecule(s) does it transport?

Answer 20

Apical surface of intestine. Depends on Na+. Transports both glucose and galactose.

Question 21

Where is SGLT-2 found? What molecule(s) does it transport?

Answer 21

Apical membrane of renal tubules. Only glucose

Question 22

Specific phosphodiesterases will inactivate which important second messengers?

Answer 22

cAMP and cGMP—they both have phosphodiester bonds

Question 23

Where is GLUT 5 found?

Answer 23

Intestine, testes, sperm - fructose

Question 24

Where is GLUT 7 found? Is the Km high or low?

Answer 24

ER of liver and kidneys. High Km

Question 25

What do high Km and low Km mean in context of glucose uptake?

Answer 25

High Km assures that glucose rapidly enters cells only in times of excess.

Low Km assures that tissues that need glucose preferentially get it first.

Question 26

Between hexokinase and glucokinase, which enzyme has a high Km?

Answer 26

Glucokinase - present in liver and needs to trap glucose only when glucose levels are high

Question 27

Specific phosphodiesterases will inactivate which important second messengers?

Answer 27

cAMP and cGMP—they both have phosphodiester bonds.

Question 28

How is erectile dysfunction treated?

Answer 28

Keep cGMP levels high. We do this by inhibiting phosphodiesterases. PDE5
inhibitors inhibit the degradation of cGMP in erectile tissue, eyes and pulmonary
vasculature

Question 29

What amino acid is nitric oxide (NO) synthesized from in the body?

Answer 29

Arginine

Question 30

What is the function of NO?

Answer 30

NO is the endothelium-derived relaxing factor, which causes vasodilation by relaxing
vascular smooth muscle. Function of nitric oxide (NO) include: relaxes smooth muscle,
neurotransmitter, prevents platelet aggregation, macrophage function

Question 31

How do nitrates decrease blood pressure?

Answer 31

Nitrates (specifically nitroglycerin) is metabolized to NO which causes relaxation of
vascular smooth muscle and therefore lowers BP

Question 32

How do nitroglycerines (nitrates) treat angina pain? (What is the correct signal cascade that corresponds to nitric oxide relaxing vascular smooth muscle, leading to dilation of blood vessels)?

Answer 32

Nitroglycerine is metabolized to NO.

Increase in NO –> increase in guanylyl cyclase –> increase in cGMP –> increase in PKG –> increase in MLCP –> MLCP removes phosphate from Myosin-P –>decrease in Myosin-P b/c it cannot cross-link with actin –> smooth muscle relaxes, so blood vessels dilate

Question 33

Irreversible inhibition of cyclooxegenase by aspirin is due to what chemical modification?

Answer 33

Acetylation

Question 34

How are the proteins of the blood clotting pathway and RAAAS pathway activated?

Answer 34

Proteolysis