The reaction of 1-aminocyclopropane-1-carboxylate synthase with a substrate analog, Se-adenosyl-L-selenomethionine.

摘要

The reaction specificity of 1-amino-1-cyclopropanecarboxylate synthase is dictated in large part by interactions with the sulfonium pole of its substrate, S-adenosylmethionine (SAM) [McCarthy et al (2001) Biochemistry 40, 12276-84]. It is shown here that this enzyme also reacts with the seleno analog of SAM, Se-adenosylselenomethionine (SeAM). The kcat/Km value of 6.0 x 105 M-1s-1 is very close to that of SAM (9.7 x 105 M-1s-1) [White et al (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 12428-12432]. The detailed reactions and rate-determining steps however, differ significantly. The SAM reaction is completely diffusion-controlled [Li et al 1997, Biochemistry, 36, 15477-88], but that with SeAM reaction is only 30% diffusion-controlled. The quinonoid intermediate, which accumulates in the steady state in the SAM reaction, cannot be observed in the corresponding reaction with SeAM, nor can it be detected under presteady stale conditions. There is no significant kinetic Calpha-hydrogen isotope effect with either substrate, but the apparent rate constant for formation of the quinonoid in the SAM reaction is decreased from 95s-1 to 75s-1 by the heavier isotope. The rate-determining steps for the SAM reaction kcat and kcat/Km values are determined by elimination of 5'-methylthioadenosine and substrate association respectively, while they are defined by the transaldimination reactions for SeAM.