Abstract
"The catalytic activity of thermophilic enzymes is low or absent at moderate temperatures at which conventional enzymes of similar function are optimally active.” Thus T. D. Brock, on what on the face of it is a surprising — but well-characterized — fact. An understanding of why this should be so has been elusive. On page 496 of this issue, Kohen et al. furnish evidence for an explanation that centres on a reduction in thermophilic (>60 °C) enzyme flexibility at mesophilic (<40 °c) temperatures.
Enzyme active sites are conserved, as are chemical mechanisms, so one would expect that homologous enzymes catalyzing the same chemical reaction would do so at about the same rate at any given temperature. But they don't1,2,3. One proposed explanation is that thermophilic enzymes might be cold-denaturing at lower temperatures; the crystallization and structure determination5,6 of many such enzymes at normal temperatures has, however, ruled that out. Another is that the mechanism changes in such a way that the rate-determining step is very different at different temperatures for enzymes isolated from thermophilic and mesophilic organisms; given the similarity in three-dimensional active-site structure between enzymes from both types of organism, this explanation also seems unlikely.