BCH 4054 PRE-TEST 2 GROUP October 16, 1996 NAME _____________________ This test is take-home and open book, and it is intended that all members of the group contribute to completing it. Only one copy is to be submitted by the group, and all members who participated should sign their names below. Test is due by 1:30 pm on Monday, October 21. __________________________ __________________________ __________________________ __________________________ __________________________ __________________________ (5) 1. Covalent modification of a protein by phosphorylation as a means of regulating enzymatic activity was first discovered with enzymes of glycogen metabolism, but is now known to regulate many cellular enzymatic reactions. Hormonal stimulation of liver by glucagon results in the phosphorylation of the following proteins. Indicate in the blank to the left whether the phosphorylated form of the indicated enzyme is the active or inactive form of the enzyme: ______ phosphorylase ______ phosphofructokinase 2 ______ phosphorylase kinase ______ glycogen synthetase ______ fructose-2,6-bisphosphatase (3) 2. Muscle is not gluconeogenic and therefore lacks glucose-6- phosphatase. What role is therefore played by fructose-1,6- bisphosphatase in muscle? (4) 3. Compare sucrose and starch synthesis in plants with respect to (a) location in the cell and (b) activated sugar precursor. (12) 4. Following is the overall reaction catalyzed by the Calvin-Benson cycle: CO2 + 3ATP + 2NADPH ---> 1/3 glyceraldehyde-3-P + 3ADP + 2NADP+ Give the structures of reactants and products for the step or steps of the cycle which: (a) Incorporate(s) CO2 into an organic form. (b) Use(s) ATP as a substrate. (c) Use(s) NADPH as a substrate. (6) 5. Tropical plants have an additional reaction for converting CO2 into an organic compound. Give the reactants and products of this reaction, and explain what it accomplishes and why it is important. (9) 6. Carbon-carbon bond formation is necessary in anabolic pathways to build up large structures from small molecules. For each of the following enzymes, give the structure of the substrate(s) and product(s) (nucleotides may be abbreviated), and indicate how energy is provided to help make the reaction energetically favorable. (a) á-ketoacyl-ACP synthase (fatty acid synthesis) (b) HMG-CoA synthase (sterol synthesis) (c) prenyl transferase (sterol synthesis) (8) 7. You are preparing radioactive intermediates of the cholesterol biosynthetic pathway by perfusing liver with [1-14C]acetate (i.e., the carboxyl carbon labeled). Give the structures of the following intermediates, circling the atoms of each which should be labeled with 14C. (a) mevalonic acid (b) dimethylallyl pyrophosphate (c) geranyl pyrophosphate (d) squalene (12) 8. For the four plasma lipoproteins, (a) chylomicrons, (b) VLDL, (c) LDL, and (d) HDL, put the letter or letters corresponding to the lipoprotein(s) for which the following statements are true in the blank to the left of the statement: ________ Contains apoprotein B-100. ________ Contains apoprotein B-48. ________ Contains apoprotein A-1. ________ Contains apoprotein C-1. ________ Source of cholesterol for tissues. ________ Transports dietary triacylglycerols. ________ Transports endogenous (made in liver) triacylglycerols. ________ Is degraded by lipoprotein lipase. ________ Is taken up by cells via receptor-mediated mechanisms. ________ Is a precursor of LDL. ________ May remove cholesterol from cells. ________ Is acted on by the enzyme LCAT. (8) 9. Glutamine synthetase is regulated in bacteria both by allosteric end-product inhibition and by covalent modification. (a) What type of covalent modification is involved? (b) Is the modified enzyme the active or inactive form? (c) The state of the enzyme (modified or unmodified) depends on the concentrations of glutamine and of alpha-ketoglutarate. Explain how these compounds regulate the modification of the enzyme. (9) 10. PRPP is the abbreviation for the "activated" form of ribose utilized in N-glycoside bond formation. Identify the following reactions in which an N-glycoside is formed by giving all the reactants and products involved in the reaction. (Names or structures are acceptable.) (a) Formation of the initial nitrogen-containing intermediate in purine biosynthesis. (b) Formation of the initial N-glycoside bond in pyrimidine biosynthesis. (c) Reaction of the nucleotide salvage pathway deficient in Lesh- Nyhan syndrome. (6) 11. Give each of the following reaction pathways, giving reactions and products of each step, but structures are not necessary. (a) Conversion of UMP to CTP (b) Conversion of CTP to dCTP (d) Conversion of AMP to dATP (6) 12. Describe how ATP affects GTP synthesis and GTP affects ATP synthesis. (6) 13. Identify the following reactions of pyrimidine biosynthesis by giving all of the reactants and products for each reaction (names or structures are okay): (a) the two steps involving glutamine as a substrate. (b) the step at which ribose becomes part of the molecule. (6) 14. The enzyme AMP deaminase converts AMP to IMP and NH4+. If this reaction is coupled to the two steps in which synthesis of AMP from IMP occurs (i.e., IMP ----> intermediate -----> AMP), the sum of the three reactions forms a cycle. Write out these three reaction steps, showing other reactants and products (structures not necessary), and then give the overall reaction catalyzed by the cycle. (This is called the purine nucleotide cycle, and is believed to play an important metabolic role in muscle.)