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Introduction

Structure of the Procaspase-7

In the cytoplasm, caspases are not already, constitutively present in their active form. They exist as free cytoplasmic inactive precursors called procaspases.

Procaspase-7 is a homodimeric globular-303 amino-acids long polypeptide. This protein contains two monomers (blue and green), representing two catalytic units. Each of these monomers is composed by a central 6-stranded β-sheet and 5 α-helices, forming a large (20 kDa) and a small (11 kDa) subunit, linked by a highly flexible interdomain. The homodimerization is performed thanks to hydrophobic interactions between the 6 β-strands of each monomer. This homodimer is organized in a “open α/β barrel fold”.

Four loops (L1 to L4), located at the two opposite ends of the β –sheet, emanate from each homodimer and define the shape of the catalytic groove of each monomer.

L2 interdomain loop links the two the large and small subunit of each procaspase-7 monomer. In the procaspase-7, this loop is in a “closed” conformation that precludes any possibility of substrate or inhibitor binding to the yet incomplete active site. Thus, in this considered conformation, the enzyme is yet inactive.

This highly flexible loop contains two cleavage sites (Asp198 - Asp207) which are essential to the maturation of the Procaspase-7 (cf MATURATION). It also contains an essential residue needed for the catalytic activity of the Caspase-7 :

is a part of the large subunit, while L3 and L4 belong to the small subunit of each monomer. These three loops will also participate in the formation of the catalytic site.

The 23 last amino acids of the N-ter extremity of procaspase-7 define a “prodomain”. This prodomain is apparently implicated in an inhibitory mechanism that maintains the procaspase (or caspase) catalytically inactive until it is cleaved. The mechanism by which the prodomain could inhibit caspase-7 enzymatic activity is still unclear.

Maturation

Structure of the Caspase-7

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