Name: _____________________________
Biological Sciences 4087
Exam II
Total: 100 points
1.(15pts) A. A plot of fractional saturation vs. pO2 for hemoglobin is (CIRCLE
ONE): HYPERBOLIC SIGMOIDAL
B. Name the O2-binding prosthetic group in hemoglobin and myoglobin.
heme or iron protoporphyrin IX
C. Explain how electrostatic bonds are important in cooperative O2 binding in hemoglobin.
In the T (tense) state, there are electrostatic bonds between the 4 subunits of hemoglobin. These hold the F helix such that the iron atom is pulled out of the plane of the heme, resulting in low affinity for O2. If O2 binds at one heme, some of these electrostatic bonds are broken. This makes it easier for subsequent O2 to bind. In the R (high affinity) state, all of these bonds are broken and the hemes bind O2 with the same affinity as myoglobin.
D. What effect does CO2 have on O2 binding in hemoglobin? In your answer, describe or draw how CO2 binds to hemoglobin.
CO2 decreases the affinity of hemoglobin for O2. It binds to an a-amino group and forms a carbamate, which stabilizes the T state of hemoglobin.
2.(3pts) A transition state analog is synthesized for a particular enzyme. What is the likely effect if that transition state analog were added to the reaction of the enzyme with its substrate? (CIRCLE ONE)
ACTIVATE THE ENZYME |
NO EFFECT |
INHIBIT THE ENZYME |
3.(6pts) Draw a hydropathy plot for the b-adrenergic receptor, which has 7 transmembrane segments. Be sure to label the axes.
See Fig. 12.27B. There are 7 highly hydrophobic regions of about 20 amino acids. 4.(9pts) Answer the following questions for the graph below. Show your calculations and include units.
A. Calculate Km
-10 = -1/Km
Km = 0.1 mM B. Calculate Vmax
0.3 = 1/ VmaxVmax = 3.3 mM/sec
C. If the total enzyme concentration in this assay is 6 mM, what is kcat?
Vmax = kcat [Etotal]
kcat = Vmax/[Etotal]
kcat = (3.3 mM/sec)/6mM
kcat = 0.55/sec
D. Draw a line on the graph to indicate the kinetics in the presence of a noncompetitive inhibitor.
5.(6pts) Fill in the blanks.
A. cAMP activates protein kinase A by binding to its regulatory subunits.
B. Protein kinase A phosphorylates serines and threonines on target proteins using
ATP as the source of phosphate.
C. The phosphates are removed from phosphorylated proteins by enzymes called
phosphatases.
6.(6pts) Draw the structure of a 20:3 (D8,11,14) fatty acid.
A carboxylic acid with 20 carbons and 3 cis double bonds
7.(8pts) A. Draw or describe a V vs. [aspartate] plot for aspartate transcarbamoylase.
The plot is sigmoidal; see Fig. 10.3
B. Draw or describe the effect on the plot if CTP is added to the assay of aspartate transcarbamoylase.
CTP is an allosteric inhibitor of ATCase and shifts the curve to the right. (It stabilizes the T state.)
8.(6pts) Calculate the DG for K+ transport from outside the cell to inside the cell if [K+] inside the cell is 150 mM and [K+] outside the cell is 5 mM and the membrane potential is -60 mV (inside negative).
R = 8.315 x 10-3 kJ/degree-mole
T = 310 oK
F = 96.5 kJ/V-mole
DG = RT ln c2/c1
+ zFDV
DG = (8.315 x 10-3 kJ/degree-mole)(310 oK) ln (150 x 10-3 M)/(5 x 10-3 M) +
(+1)(96.5 kJ/V-mole)(-0.06 V)
DG = +2.98 kJ/mole
9.(10pts) A. The Kt for the pancreatic GLUT 2 transporter is 20 mM. At physiological blood glucose (4-8 mM), the rate of glucose transport is (CIRCLE ONE):
|
LINEARLY PROPORTIONAL TO THE BLOOD
GLUCOSE CONCENTRATION |
NEARLY CONSTANT AND EQUAL TO THE Vmax |
|
B. Lactose transport in E. coli is driven by the H+ gradient across the cell membrane. This is called (CIRCLE ONE):
|
PRIMARY ACTIVE TRANSPORT |
SECONDARY ACTIVE TRANSPORT |
C. Briefly describe the mechanism of Ca2+ transport by SERCA.
Ca2+ binds on the cytoplasmic side of the transporter. ATP phosphorylates
the transporter, causing a conformational change in the transporter. Ca2+ is released
to the inside of the sarcoplasmic reticulum. The phosphate bond is
hydrolyzed and the transporter returns to its original conformation.
D. Describe the structure that senses the change in membrane potential in the voltage-gated K+ channel.
The S4 segment forms an a helix with positively charge lysines and arginines.
10.(12pts) Fill in the blanks.
A. NO synthase makes NO from the amino acid arginine
B. NO activates the soluble enzyme guanylate cyclase.
C. This enzyme synthesizes the signaling molecule cGMP.
.D. Epinephrine binding to the b-adrenergic receptor activates the heterotrimeric protein
Gs.
E. Name 2 off-switches or methods of down-regulation of the b-adrenergic receptor pathway.
dissociation of epinephrine; phosphorylation of the receptor, binding of b-arrestin, and endocytosis of the receptor; conversion of cAMP to AMP by phosphodiesterase; phosphatases
11.(3pts) Identify the structure shown at right.
phosphatidylinositol
12.(16pts) Define:
A. v-Src- a viral protein tyrosine kinase that is similar to a normal cellular tyrosine kinase except that it lacks a C-terminal tyrosine residue. This residue is phosphorylated to inactivate the normal cellular protein. v-Src, on the other hand, is always active and causes abnormal cell proliferation
B. apoenzyme- the protein part of an enzyme without its cofactor
C. zymogen- an inactive precursor of an enzyme
D. GPI- glycosylphosphatidylinositol, a lipid anchor for membrane proteins on the outside surface of cells