Structural and mechanistic studies of the orf12 gene product from the clavulanic acid biosynthesis pathway

Structural and biochemical studies of the orf12 gene product (ORF12) from the clavulanic acid (CA) biosynthesis gene cluster are described. Sequence and crystallographic analyses reveal two domains: a C-terminal penicillin-binding protein (PBP)/β-lactamase-type fold with highest structural similarity to the class A β-lactamases fused to an N-terminal domain with a fold similar to steroid isomerases and polyketide cyclases. The C-terminal domain of ORF12 did not show β-lactamase or PBP activity for the substrates tested, but did show low-level esterase activity towards 3'-O-acetyl cephalosporins and a thioester substrate. Mutagenesis studies imply that Ser173, which is present in a conserved SXXK motif, acts as a nucleophile in catalysis, consistent with studies of related esterases, β-lactamases and D-Ala carboxypeptidases. Structures of wild-type ORF12 and of catalytic residue variants were obtained in complex with and in the absence of clavulanic acid. The role of ORF12 in clavulanic acid biosynthesis is unknown, but it may be involved in the epimerization of (3S,5S)-clavaminic acid to (3R,5R)-clavulanic acid.

Spectroscopic studies on Arabidopsis ETHE1, a glyoxalase II-like protein

ETHE1 (ethylmalonic encephalopathy protein 1) is a beta-lactamase fold-containing protein that is essential for the survival of a range of organisms. In spite of the apparent importance of this enzyme, very little is known about its function or biochemical properties. In this study Arabidopsis ETHE1 was over-expressed and purified and shown to bind tightly to 1.2+/-0.2 equivalents of iron. (1)H NMR and EPR studies demonstrate that the predominant oxidation state of Fe in ETHE1 is Fe(II), and NMR studies confirm that two histidines are bound to Fe(II). EPR studies show that there is no antiferromagnetically coupled Fe(III)Fe(II) center in ETHE1. Gel filtration studies reveal that ETHE1 is a dimer in solution, which is consistent with previous crystallographic studies. Although very similar in terms of amino acid sequence to glyoxalase II, ETHE1 exhibits no thioester hydrolase activity, and activity screening assays reveal that ETHE1 exhibits low level esterase activity. Taken together, ETHE1 is a novel, mononuclear Fe(II)-containing member of the beta-lactamase fold superfamily.