The organophosphate degrading (opd) gene cluster of plasmid pPDL2 of Flavobacterium sp. ATCC27551 contains a novel open-reading frame, orf243. This was predicted to encode an alpha/beta hydrolase distantly related to the meta-fission product (MFP) hydrolases such as XylF, PhnD, and CumD. By homology modeling Orf243 has most of the structural features of MFP hydrolases including the characteristic active site catalytic triad. The purified protein (designated MfhA) is a homotetramer and shows similar affinity for 2-hydroxy-6-oxohepta-2,4-dienoate (HOHD), 2-hydroxymuconic semialdehyde (HMSA), and 2-hydroxy-5-methylmuconic semialdehyde (HMMSA), the meta-fission products of 3-methyl catechol, catechol, and 4-methyl catechol. The unique catalytic properties of MfhA and the presence near its structural gene of cis-elements required for transposition suggest that mfhA has evolved towards encoding a common hydrolase that can act on meta-fission products containing either aldehyde or ketone groups.
Title: A series of crystal structures of a meta-cleavage product hydrolase from Pseudomonas fluorescens IP01 (CumD) complexed with various cleavage products Fushinobu S, Jun SY, Hidaka M, Nojiri H, Yamane H, Shoun H, Omori T, Wakagi T Ref: Biosci Biotechnol Biochem, 69:491, 2005 : PubMed
Meta-cleavage product hydrolase (MCP-hydrolase) is one of the key enzymes in the microbial degradation of aromatic compounds. MCP-hydrolase produces 2-hydroxypenta-2,4-dienoate and various organic acids, according to the C6 substituent of the substrate. Comprehensive analysis of the substrate specificity of the MCP-hydrolase from Pseudomonas fluorescens IP01 (CumD) was carried out by determining the kinetic parameters for nine substrates and crystal structures complexed with eight cleavage products. CumD preferred substrates with long non-branched C6 substituents, but did not effectively hydrolyze a substrate with a phenyl group. Superimposition of the complex structures indicated that benzoate was bound in a significantly different direction than other aliphatic cleavage products. The directions of the bound organic acids appeared to be related with the k(cat) values of the corresponding substrates. The Ile139 and Trp143 residues on helix alpha4 appeared to cause steric hindrance with the aromatic ring of the substrate, which hampers base-catalyzed attack by water.
