BCH 4053                        HOUR TEST 2          NAME _____________________
 June 28, 1996
          
Points

(12) 1.   An enzyme which follows the simple Michaelis-
          Menten rate law shows the following kinetic
          constants for two different substrates which
          have related but slightly different structures:

          Substrate       Km             Vm

              A      0.061 æM      35 æmol-min-1-mg-1

              B       2.6  æM     195 æmol-min-1-mg-1


          (a)  Using the Michaelis-Menten equation determine the velocity of
               reaction of each substrate at a concentration of 0.015 æM.













          (b)  Calculate the velocity of reaction of each substrate at a
               concentration of 1.5 æM.











(6)  2.   The pH-rate profile for ribonuclease is bell shaped, with a pH
          maximum at 6.0 and inflection points of approximately 5.8 and 6.2 in
          the two sides of the curve.  
          (a)  What amino acid side chain(s) would be likely candidates for
               titration at these pK values?


          (b)  Presumably the groups responsible for these pH effects would be
               involved in general-acid or general-base catalysis in the
               mechanism.  Identify by pK (5.8 or 6.2) the

               ______     general-acid catalyst

               ______     general-base catalyst


(8)  3.   Which of the following are not assumptions made in the derivation of
          the Michaelis-Menten equation for a one-substrate, one-product
          reaction?  (Put a check by the inappropriate assumptions.)

          ____ [S] >> [Etotal].

          ____ The rate of formation of ES equals its rate of breakdown.

          ____ The rate of breakdown of ES to E + P is much faster than the
               rate of breakdown of ES back to E + S.

          ____ The velocity of the catalyzed reaction is equal to k1[E][S].

          ____ The concentration of ES is small relative to the concentration
               of Etotal.

          ____  [Etotal]  >>  [Efree].

          ____ The rate of reaction is equal to k2[ES].

          ____ KM = [E][S]/[ES]



(9)  4.   Aspartate transcarbamoylase shows a sigmoid rate law when plotting
          velocity versus aspartate concentration.  The explanation for this
          kinetic behavior is similar to that for the binding curve of
          hemoglobin.  The enzyme exists in two conformations:  a taut (T)
          conformation which is inactive and a relaxed (R) conformation which
          is active.  For each of the following substances which bind to the
          enzyme, indicate whether binding is to the catalytic or regulatory
          subunit, preferentially to the R or T conformation, and whether the
          effect of binding is considered a homotropic or heterotropic effect.

          Ligand           Subunit        Conformation         Effect

          aspartate       ________       ___________          _________

          ATP             ________       ___________          _________

          CTP             ________       ___________          _________


(6)  5.   What amino acid side chains make up the catalytic triad of
          chymotrypsin?






(6)  6.   A competitive inhibitor binds to (circle correct answer)

               E    ES     Both

          while an uncompetitive inhibitor binds to (circle correct answer)

               E    ES     Both.
               
               
(5)  7.   Why are transition state analogues more effective inhibitors than
          substrate analogues?







(6)  8.   In the hydrolysis of a peptide bond by a protease such as
          chymotrypsin, how does the geometry of the transition state differ
          from the geometry of the substrate in the portion of the molecule
          where hydrolysis occurs?







(9)  9.   Draw out the structure and give the shorthand abbreviation for the
          following fatty acids:

          linoleic acid



          arachidonic acid



          palmitic acid




(6)  10.  Give the chemical reaction for the saponification of fat, showing
          structures of reactants and products.









(6)  11.  Phospholipase C is an important enzyme involved in cellular
          regulation.  It cleaves phosphatidyl inositol diphosphate (PIPP) into
          what two fragments?  (Names of the fragments are okay).








(9)  12.  Following are some statements about lipids.  Identify the lipid or
          lipids referred to by putting the appropriate letter or letters from
          the key list of lipids in the blank to the left of the statement.

          _________________________  Contains glycerol.

          _________________________  Contains sphingosine.

          _________________________  Contains phosphate.

          _________________________  An essential fatty acid.

          _________________________  Contains nitrogen

          _________________________  A glycolipid

          Key list:

               a.   cholesterol               h.    stearic acid
               b.   lecithin                  i.    ceramide
               c.   sphingomyelin             j.    phosphatidyl serine
               d.   phosphatidyl glycerol     k.    triglyceride
               e.   PE                        l.    plasmalogen
               f.   vitamin K                 m.    phosphatidic acid
               g.   ganglioside               n.    linoleic acid


(6)  13.  What do the terms fluid and mosaic refer to in the fluid mosaic model
          of membrane structure?













(6)  14.  The transport of sodium and glucose into a cell, driven by the
          concentration gradient of sodium ions, is an example of cotransport

          known as ____________ and is a form of _______________ (primary or
          secondary?) active transport.

          How is the sodium gradient produced?