Fumarase, rate constant

In the Briggs-Maldane mechanism, when k2 is much greater than k-i, kcJKM is equal to kx, the rate constant for the association of enzyme and substrate. It is shown in Chapter 4 that association rate constants should be on the order of 108 s l M l. This leads to a diagnostic test for the Briggs-Haldane mechanism the value of kaJKu is about 107 to 108 s-1 M-1. Catalase, acetylcholinesterase, carbonic anhydrase, crotonase, fumarase, and triosephosphate isomerase all exhibit Briggs-Haldane kinetics by this criterion (see Chapter 4, Table 4.4). 

At pH 6, the enzyme fumarase (50 kDa/subunit) converts fumarate to malate, and has a soluble Km for fumarate of 5.7 pM. The dissociation rate constant for the enzyme-substrate complex into enzyme plus malate has been measured as 1.45 x lO -1. 

An excellent study has been made of the temperature dependence of the steady-state kinetic parameters of the fumarase reaction [6]. By studying the temperature dependence over a wide range of pH, the apparent activation energies and standard-enthalpy changes associated with the pH-independent steady-state parameters, the lower bounds of the rate constants, and the ionization constants of the groups at the active site were obtained. The results are summarized in Table 9-1. In this case the temperature dependence of all parameters appears quite normal. The standard-enthalpy changes of... 

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