STRUCTURE-BASED DESIGN OF A LYSOZYME WITH ALTERED CATALYTIC ACTIVITY

Here we show that the substitution Thr 26 --> His in the active site o f T4 lysozyme causes the product to change from the alpha- to the beta -anomer. This implies an alteration in the catalytic mechanism of the enzyme. From the change in product, together with inspection of releva nt crystal structures, it is inferred that wild-type T4 lysozyme is an anomer-inverting enzyme with a single displacement mechanism in which water attacks from the a-side of the substrate. In contrast, the muta nt T26H is an anomer-retaining enzyme with an apparently double displa cement mechanism in which a water molecule attacks from the opposite s ide of the substrate. The results also show that the mechanism of wild -type T4 lysozyme differs from that of hen egg-white lysozyme even tho ugh both enzymes are presumed to have evolved from a common precursor