Substrate Binding and Catalysis by Ubiquitin C-Terminal Hydrolases: Identification of Two Active Site Residues

Abstract

Ubiquitin C-terminal hydrolases (UCH's) are a newly-defined class of thiol proteases implicated in the proteolytic processing of polymeric ubiquitin. They are important for the generation of monomeric ubiquitin, the active component of the eukaryotic ubiquitin-dependent proteolytic system. There are at least three mammalian isozymes which are tissue specific and developmentally regulated. To study the structure and functional roles of these highly homologous enzymes, we have subcloned and overexpressed two of these isozymes, UCH-L1 and UCH-L3. Here, we report their purification, physical characteristics, and the mutagenesis of UCH-L1. Site-directed mutagenesis of UCH-L1 reveals that C90 and H161 are involved in catalytic rate enhancement. Data from circular dichroic and Raman spectroscopy, as well as secondary structure prediction algorithms, indicate that both isozymes have a significant amount of α- helix (>35%), and contain no disulfide bonds. Both enzymes are reasonably stable, undergoing a reversible thermal denaturation at 52 °C. These transitions are characterized by thermodynamic parameters typical of single domain globular proteins. Substrate binding affinity to UCH-L3 was directly measured by equilibrium gel filtration (K_d = 0.5 μM), and the results are similar to the kinetically determined K_m for ubiquitin ethyl ester (0.6 μM). The binding is primarily electrostatic in nature and indicates the existence of a specific and extensive binding site for ubiquitin on the surface of the enzyme.