BCH 4054                        HOUR TEST 2          NAME _____________________
 October 25, 1996

Points

(6)  1.   Which three reactions of glycolysis are not
          used in gluconeogenesis of glucose from
          pyruvate?  (Identify by either name of enzyme
          or reaction catalyzed).













     2.   One of these steps is the primary site of regulation of glycolysis
          versus gluconeogenesis in the liver.
          
(2)       (a)  What gluconeogenic enzyme is used at this step.



(6)       (b)  Explain how metabolic flux through this step is allosterically
               controlled by cellular intermediates that signal the "energy
               state" of the cell by completing the following table.

                    Enzyme                      Inhibitor(s)         Activator(s)

               ________________________       ______________       ______________

               ________________________       _______________      ______________

(4)       (c)  Explain how metabolic flux through this step is hormonally
               regulated by glucagon, a hormone that signals the liver to
               produce and excrete more glucose.  Diagram the sequence of
               intermediates by which the hormonal signal is transmitted to the
               enzymes in the pathway that are being regulated. 
















(4)  3.   Explain why animals cannot utilize fatty acids as a source of
          glucose.  



(8)  4.   Compare and contrast glycogen synthesis in animals with starch
          synthesis in plants by filling in the following table.

                               Glycogen synthesis        Starch synthesis

          Cellular location    __________________        __________________
          Activated sugar
             substrate         __________________        __________________
          Regulation: 
            at which enzyme?   __________________        __________________
            how is enzyme
               activated?      __________________        __________________
          

(8)  5.   Transketolase is an important enzyme in the Calvin cycle.  Give the
          structure of the reactants and products of one of the transketolase
          catalyzed reactions found in this cycle.










     6.   Rubisco, the enzyme involved in "fixing" CO2 in the Calvin cycle, is
          thought to be the most abundant protein in the biosphere.

(4)       (a)  Give the structure of the substrates and products catalyzed by
               this enzyme.







(4)       (b)  Changes in pH and [Mg2+] in the stroma of the chloroplast affect
               the activity of Rubisco.  Explain how these changes allow
               Rubisco to be activated in the light and deactivated in the
               dark.








(6)  7.   Describe the roles that citrate play in the synthesis of fatty acids.








(4)  8.   In early crude extracts obtained from pigeon liver, the incorporation
          of [14C]acetate into fatty acids was stimulated by adding HCO3- to the
          extract, but if [14C]HCO3- is added, there is no incorporation of
          radioactivity from the HCO3- into the fatty acids.  Explain why.







(12) 9.   Bacterial fatty acid synthesis from acetyl-CoA and malonyl-CoA
          requires six enzymatic steps involving seven separate proteins.  Give
          the reactants and products of each step (names or structures) and the
          name of the enzyme catalyzing the step.





















(6)  10.  What are the two classes of eicosanoids, and what dietary fatty acids
          are necessary for their synthesis?



     
     
     
     
     
     11.  VLDL is converted to LDL in the blood.
(3)       (a)  What are the major changes in the composition of the lipoprotein
               particle as a result of this process?





(2)       (b)  What two enzymes are involved in this process?




(3)       (c)  How is HDL involved in this process?




(10) 12.  Adenine contains five nitrogen atoms, each derived from an amino
          acid.  Draw the structure of adenine, and with arrows indicate the
          amino acid source of each nitrogen atom.







(8)  13.  In the following table, the fifteen steps involved in the
          biosynthesis of purines are indicated by showing the key reactant and
          product of each step.  The pathway is regulated by a classical
          "feedback inhibition" by the end and branch products, IMP, AMP, and
          GMP.  Identify the sites of inhibition by putting a check mark in the
          appropriate blanks.
                                                         Inhibited by:
               Enzymatic Step                        IMP       AMP        GMP

          1.   ribose-5 phosphate--->PRPP           _____     _____      _____
          2.   PRPP--->phosphoribosylamine          _____     _____      _____
          3.   Phosphribosylamine--->GAR            _____     _____      _____
          4.   GAR--->FGAR                          _____     _____      _____
          5.   FGAR--->FGAM                         _____     _____      _____
          6.   FGAM--->AIR                          _____     _____      _____
          7.   AIR--->CAIR                          _____     _____      _____
          8.   CAIR--->SAICAR                       _____     _____      _____
          9.   SAICAR--->AICAR                      _____     _____      _____
          10.  AICAR--->FAICAR                      _____     _____      _____
          11.  FAICAR--->IMP                        _____     _____      _____
          12.  IMP--->adenylosuccinate              _____     _____      _____
          13.  IMP--->XMP                           _____     _____      _____
          14.  adenylosuccinate--->AMP              _____     _____      _____
          15.  XMP--->GMP                           _____     _____      _____