X-RAY STRUCTURE OF 5-AMINOLEVULINATE DEHYDRATASE, A HYBRID ALDOLASE

5-Aminolaevulinate dehydratase (ALAD) is a homo-octameric metallo-enzy me that catalyses the formation of porphobilinogen from 5-aminolaevuli nic acid. The structure of the yeast enzyme has been solved to 2.3 Ang strom resolution, revealing that each subunit adopts a TIM barrel fold with a 39 residue N-terminal arm. Pairs of monomers wrap their arms a round each other to form compact dimers and these associate to form a 422 symmetric octamer. All eight active sites are on the surface of th e octamer and possess two lysine residues (210 and 263), one of which, Lys 263, forms a Schiff base link to the substrate. The two lysine si de chains are close to two zinc binding sites one of which is formed b y three cysteine residues (133, 135 and 143) while the other involves Cys 234 and His 142. ALAD has features at its active site that are com mon to both metallo-and Schiff base-aldolases and therefore represents an intriguing combination of both classes of enzyme. Lead ions, which inhibit ALAD potently, replace the zinc bound to the enzyme's unique triple-cysteine site.