Model peptide substrates and ligands in analysis of action of mammalian protein disulfide-isomerase.

Protein disulfide-isomerase (PDI) polypeptide comprises four distinct but homologous domains can function alone, as homo-oligomers, or as an obligatory component of hetero-oligomeric species, such as prolyl-4-hydroxylase and microsomal triglyceride transfer protein. Thus, PDI is a complex enzyme that catalyzes a complex reaction. Advances in understanding the action of this complex enzyme have come from three directions: (1) the definition of the domain structure of the PDI polypeptide; (2) the definition and validation of “partial reactions” with simple substrates, representing specific elements of the overall reaction catalyzed by PDI on its complex physiological substrates; and (3) the combination of these inputs to express recombinant PDI constructs comprising individual domains, combinations of domains or active-site mutant species, and to characterize them in functional terms. This chapter focuses on the second of these advances, describing peptide substrates for partial reactions of PDI and their use to define the overall catalytic process and to establish the roles within it of individual domains of PDI.