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78 Chapter 4
[S]
(mM)
Rate of product formation
(nmol min 1 ; duplicate assays)
1.8 4.75, 4.44
0.9 3.55, 3.20
0.4 2.15, 2.19
0.2 1.28, 1.32
0.1 0.74, 0.76
(3) Some enzymes are inhibited in the presence of high concentrations of
substrate. This behaviour can be rationalised by the model below, involving
the formation of a non-productive ES 2 complex. Starting from this model,
use the steady state approximation for ES and ES 2 to construct a rate
equation of the form given below. By considering the behaviour at low
and high substrate concentrations, sketch the expected dependence of
v versus [S].
E + S
k 1 k 3
ES
k -1
k -2 k 2
E + P
ES 2
Rate =
k 3 [E] 0 [S]
where K 2 = k 2 /k -2
K M + [S] + K 2 [S] 2
(4) A carbon–phosphorus lyase activity has been found which catalyses the
reductive cleavage of ethyl phosphonate to ethane, as shown below. The
stereochemistry of this reaction was elucidated using a chiral methyl group
approach. Incubation of [1R- 2 H, 3 H]-ethyl phosphonate with enzyme gave a
3 H-labelled ethane product, which was converted via halogenation and
oxidation into 3 H-labelled acetic acid. Analysis using the method described
in the text revealed that the acetate derivative had predominantly the
S conWguration. Incubation of [1S- 2 H, 3 H]-ethyl phosphonate with
enzyme and analysis by the same method gave 2R-acetate. Deduce whether
the reaction proceeds with retention or inversion of conWguration, and
comment on this result.