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

L2 interdomain loop links the large and small subunit of each monomer of the procaspase-7. In the procaspase-7, this loop is in a “closed” conformation that precludes any possibility of substrate or inhibitor binding to the thus-incomplete active site. Thus, in this considered procaspase-7 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 as well an essential residue needed for the catalytic activity of the Caspase-7 : Cys186

L1 is a part of the large subunit, while L3 and L4 belong to the small subunit of each monomer. These three loops will participate in the formation of the catalytic site once the Caspase-7 is in its active form.

The 23 last amino acids of the N-term extremity of procaspase-7 define a “prodomain”. This prodomain is apparently implicated in an inhibitory mechanism that maintains the Procaspase (or the 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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