Folding and Stability of Class A beta-Lactamases

Class A β-lactamases have proved useful as model proteins in studying a wide
variety of aspects of protein folding. We review those features that have shed light on
kinetic intermediates that take part in folding, including some insight into the molecular
basis of the kinetic and stable molten-globule states that have been identified. The
contrast between the folding behaviour of PC1 and the two lactamase mutants, P54 and
P2, can be attributed in some detail to changes in molecular conformation. The early,
very rapid stages of folding of β-lactamases have been shown to be multiphasic, and an
interesting intermediate is described that has non-native contacts involving burial of the
C-terminal tryptophan. A further feature is that the region around the disulphide bond in
the TEM enzymes is formed very early in the foldingreaction. The unusual feature of
class A β-lactamases, i.e. a cispeptide bond critical in the stereochemistry of the active
site that usually, but not always, involves a proline residue, has been shown to be
important in accounting for the slow folding reactions.
The effect of substrates on the stabilization of the enzymes and on their reversible
deactivation is also reviewed.