3ah3

Publication Abstract from PubMed

HICDH (homoisocitrate dehydrogenase), which is involved in lysine biosynthesis through alpha-aminoadipate, is a paralogue of IPMDH [3-IPM (3-isopropylmalate) dehydrogenase], which is involved in leucine biosynthesis. TtHICDH (Thermus thermophilus HICDH) can recognize isocitrate, as well as homoisocitrate, as the substrate, and also shows IPMDH activity, although at a considerably decreased rate. In the present study, the promiscuous TtHICDH was evolved into an enzyme showing distinct IPMDH activity by directed evolution using a DNA-shuffling technique. Through five repeats of DNA shuffling/screening, variants that allowed Escherichia coli C600 (leuB) to grow on a minimal medium in 2 days were obtained. One of the variants LR5-1, with eight amino acid replacements, was found to possess a 65-fold increased k(cat)/K(m) value for 3-IPM, compared with TtHICDH. Introduction of a single back-replacement H15Y change caused a further increase in the k(cat)/K(m) value and a partial recovery of the decreased thermotolerance of LR5-1. Site-directed mutagenesis revealed that most of the amino acid replacements found in LR5-1 effectively increased IPMDH activity; replacements around the substrate-binding site contributed to the improved recognition for 3-IPM, and other replacements at sites away from the substrate-binding site enhanced the turnover number for the IPMDH reaction. The crystal structure of LR5-1 was determined at 2.4 A resolution and revealed that helix alpha4 was displaced in a manner suitable for recognition of the hydrophobic gamma-moiety of 3-IPM. On the basis of the crystal structure, possible reasons for enhancement of the turnover number are discussed.

Enhancement of the latent 3-isopropylmalate dehydrogenase activity of promiscuous homoisocitrate dehydrogenase by directed evolution.,Suzuki Y, Asada K, Miyazaki J, Tomita T, Kuzuyama T, Nishiyama M Biochem J. 2010 Oct 11;431(3):401-10. PMID:20735360^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.