1lbk

From Proteopedia

Revision as of 20:45, 2 May 2008

Template:STRUCTURE 1lbk

Crystal structure of a recombinant glutathione transferase, created by replacing the last seven residues of each subunit of the human class pi isoenzyme with the additional C-terminal helix of human class alpha isoenzyme

Overview

We have sought the structural basis for the differing substrate specificities of human glutathione transferase P1-1 (class Pi) and human glutathione transferase A1-1 (class Alpha) by adding an extra helix (helix 9), found in the electrophilic substrate-binding site (H-site) of the human class Alpha enzyme, at the C terminus of the human class Pi enzyme. This class Pi-chimera (CODA) was expressed in Escherichia coli, purified and characterized by kinetic and crystallographic approaches. The presence of the newly engineered tail in the H-site of the human Pi enzyme alters its catalytic properties towards those exhibited by the human Alpha enzyme, as assessed using cumene hydroperoxide (diagnostic for class Alpha enzymes) and ethacrynic acid (diagnostic for class Pi) as co-substrates. There is a change of substrate selectivity in the latter case, as the k(cat)/K(m)(EA) value decreases about 70-fold, compared to that of class Pi. With 1-chloro-2,4-dinitrobenzene as co-substrate there is a loss of catalytic activity to about 2% with respect to that of the Pi enzyme. Crystallographic and kinetic studies of the class Pi-chimera provide important clues to explain these altered catalytic properties. The new helix forms many complimentary interactions with the rest of the protein and re-models the original electrophilic substrate-binding site towards one that is more enclosed, albeit flexible. Of particular note are the interactions between Glu205 of the new tail and the catalytic residues, Tyr7 and Tyr108, and the thiol moiety of glutathione (GSH). These interactions may provide an explanation of the more than one unit increase in the pK(a) value of the GSH thiolate and affect both the turnover number and GSH binding, using 1-chloro-2,4-dinitrobenzene as co-substrate. The data presented are consistent with the engineered tail adopting a highly mobile or disordered state in the apo form of the enzyme.

About this Structure

1LBK is a Single protein structure of sequence from Homo sapiens. Full crystallographic information is available from OCA.

Reference

Engineering a new C-terminal tail in the H-site of human glutathione transferase P1-1: structural and functional consequences., Micaloni C, Kong GK, Mazzetti AP, Nuccetelli M, Antonini G, Stella L, McKinstry WJ, Polekhina G, Rossjohn J, Federici G, Ricci G, Parker MW, Lo Bello M, J Mol Biol. 2003 Jan 3;325(1):111-22. PMID:12473455 Page seeded by OCA on Fri May 2 23:45:42 2008