BCH 4053                          Mini-Exam             NAME _____________________
 August 2, 1996
      
     Answer either question 1 or 2
Points

(25) 1.   Give the step-wise mechanism catalyzed by the
          enzyme pyruvate dehydrogenase, showing the
          partial structure of all the enzyme-bound
          intermediates.  Identify the protein components
          catalyzing each step and the coenzymes involved
          as either cosubstrates or prosthetic groups.

(25) 2.   Glucagon is a peptide hormone produced in
          response to low blood glucose.  Binding of
          glucagon to a receptor in the membrane of the
          liver causes a number of reactions to occur in
          the liver which result in covalent modification of several enzymes. 
          Diagram the series of intermediate steps between glucagon binding and
          enzyme covalent modification, indicate which enzymes we have
          discussed that are modified and how the modification changes their
          activity, and explain the overall consequence of these modifications
          on the metabolism of liver glycogen.  How is the effect of glucagon
          reversed when blood glucose levels increase?  How is glycogen
          breadown in liver stimulated in the absence of hormonal stimulation? 
          
          
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     Answer either question 3 or 4


(25) 3.   Starting with [1-14C]-glucose, trace the radioactivity through
          glycolysis and into the oxaloacetate formed after the first turn of
          the citric acid cycle.  To do so, give the structure of each
          intermediate in the pathway, and circle the carbon atom(s) of each
          intermediate which are derived from [1-14C]-glucose.

(25) 4.   To demonstrate how the pentose-phosphate pathway can convert pentoses
          to hexoses, show a sequence of steps by which ribose-5-phosphate can
          be converted to fructose-6-phosphate.  Show the structures of the
          intermediates and the names of the enzymes catalyzing each step. 
          (You may only use intermediates derived from ribose-5-phosphate, but
          it is not necessary to convert the ribose-5-phosphate completely into
          fructose-6-phosphate, i.e. you might have other byproducts from your
          pathway).
          
















































        Answer only 2 of questions 5-8

(25) 5.   One of the proton dissociations of hemoglobin occurs near neutral pH. 
          This dissociation is affected by the binding of oxygen to Hb.  Assume
          that the dissociations for oxyhemoglobin (HbO2) and deoxyhemoglobin
          (Hb) can be treated as a simple monoprotic acids as follows:

          HHbO2  ŝ  H+  +  HbO2 , pK = 6.6;   and     HHb  ŝ  H+  +  Hb , pK = 8.2;

          (a)   Assume that blood is 2.5 mM in hemoglobin.  At the plasma pH of
                7.4, and the hemoglobin in the lungs fully oxygenated, what are
                the concentrations of the protonated (HHbO2) and unprotonated
                (HbO2) forms?
          (b)   In tissues, the pressure of O2 drops, and the oxygen dissociates
                from hemoglobin.  The deoxyhemoglobin is a stronger base, and
                must be titrated with protons for the pH to remain constant at
                7.4.  Assuming the oxygen were completely dissociated, how many
                protons would be required per liter to produce the proper Hb/HHb
                ratio for pH 7.4?







































(25) 6.   Given the following data for a peptidase hydrolysing the dipeptide
          gly.gly:

          S (mM)           1.5     2.0     3.0     4.0     8.0     16.0

          v (ĉmol/min)     0.21    0.24    0.28    0.33    0.40     0.45

          Plot these data in both the Lineweaver-Burke plot and the Eadie-
          Hofstee plot, and determine Vm and Km from both graphs.  Show your
          work below, but obtain a piece of graph paper to use for the graphs. 
          Use care in selecting your axes.  Be sure to put your name on the
          paper, and staple it to the test.




















































(25) 7.   Aldolase catalyses the following reaction of glycolysis:

          fructose-1,6-    ---->    dihydroxyacetone     +    glyceraldehyde-3-
            diphosphate                    phosphate                  phosphate

          (a)   Give the structures of the reactant and products of this
                reaction.











          (b)   delta Go' for this reaction is +23.9 kJ mol-1.  Calculate K',
                the equilibrium constant.  
                (R = 8.314x10-3 kJ-mol-1-deg-1;  T = 310 K;  ln x = 2.3log x.)

















          (c)   Calculate delta G' for the reaction when fructose-1,6-
                diphosphate is 1.0x10-4 M, dihydroxyacetone phosphate is 4.0x10-
                5 M, and glyceraldehyde-3-phosphate is 2.5 x 10-6 M.
                
                
                
                














(25) 8.   Give the structure of 6 of the following:

          (a)  phenylalanine               (b)  N-acetylgalactosamine











          (c)  phosphatidyl choline        (d)  histidine











          (e)  GTP                         (f)  biotin











          (g)  thymine                     (h)  linoleic acid