Hs. Subramanya et al., ENZYMATIC KETONIZATION OF 2-HYDROXYMUCONATE - SPECIFICITY AND MECHANISM INVESTIGATED BY THE CRYSTAL-STRUCTURES OF 2 ISOMERASES, Biochemistry, 35(3), 1996, pp. 792-802
5-Carboxymethyl-2-hydroxymuconate isomerase (CHMI) and 4-oxalocrotonat
e tautomerase (4-OT) are enzymes that catalyze the isomerization of un
saturated ketones. They share a common enzyme mechanism, although they
show a preference for different substrates. There is no apparent sequ
ence homology between the enzymes. To investigate the molecular mechan
ism and the basis for their substrate specificity, we have determined
the crystal structures of the two enzymes at high resolution. 4-OT is
hexameric, with the subunits arranged with 32 symmetry. CHMI is trimer
ic and has extensive contacts between subunits, which include secondar
y structural elements. The central core of the CHMI monomer has a fold
similar to a 4-OT dimer, but the secondary structural elements that f
orm the subunit contacts around the 3-fold axis are different in the t
wo enzymes. The region of greatest similarity between the two enzymes
is a large pocket that is proposed to be the active site, The enzymes
appear to operate via a ''one-base'' mechanism, and the possible role
of residues in this pocket is discussed in view of this idea. Finally,
the molecular basis for substrate specificity in the two enzymes is d
iscussed.