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Page 411<br />

design of inhibitors. The structure of ICE has been solved <strong>by</strong> x-ray crystallography to 2.6 Å in complex<br />

with an acetyl-Tyr-Val-Asp-H tetra-peptide [50]. Figure 8a illustrates the tertiary structure of the<br />

heterodimer with a dimension of about 45 Å × 35 Å × 25 Å. The molecule itself is oligomeric and<br />

contains two subunits, p20 (residues 120–297) and p10 (317–404) of relative molecular weight 20 kDa<br />

and 10 kDa, respectively. The two subunits form an intimately connected heterodimer. The core of ICE<br />

is a six-stranded β sheet containing 5 parallel strands and one antiparallel strand. The core is bounded <strong>by</strong><br />

six alpha helices that lie parallel to the beta sheets. Physiologically ICE occurs as a (p20) 2(p10) 2<br />

tetramer. The molecule is a cysteine protease, which has a unique preference among the mammalian<br />

proteases for cleaving bonds. It has only one known with aspartic acid adjacent and N-terminal to the P 1<br />

scissile bond. It has only one known physiological substrate, proIL-1β, which it cleaves to active IL-1β.<br />

The struc-<br />

Figure 8<br />

(a) Stereo diagrams of the heterodimer structure of Interleukin-1 Converting<br />

Enzyme (ICE). (b) The tetrapeptide inhibitor (Asp-Ala-Val-Tyr) covalently<br />

bound to Cys285 in the active site. Tetrapeptied shown in black, p20 subunit in<br />

dark gray, and p10 subunit in light gray. Produced <strong>by</strong> Molscript [106].<br />

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