POLYSACCHARIDE HYDROLASE FOLDS DIVERSITY OF STRUCTURE AND CONVERGENCEOF FUNCTION

Polysaccharide glycosyl hydrolases are a group of enzymes that hydroly ze the glycosidic bond between carbohydrates or between a carbohydrate and a noncarbohydrate moiety. Here we illustrate that traditional sch emes for grouping enzymes, such as by substrate specificity or by orga nism of origin, are not appropriate when thinking of structure-functio n relationships and protein engineering. Instead, sequence comparisons and structural studies reveal that enzymes with diverse specificities and from diverse organisms can be placed into groups among which mech anisms are largely conserved and insights are likely to be transferrab le. In particular, we illustrate how enzymes have been grouped using p rotein sequence alignment algorithms and hydrophobic cluster analysis. Unfortunately for those who seek to improve cellulase function by des ign, cellulases are distributed throughout glycosyl hydrolase Families 1,5,6,7,9, and 45. These cellulase families include members from wide ly different fold types, i.e., the TIM-barrel, beta alpha beta-barrel variant (a TIM-barrel-like structure that is imperfectly superimposabl e on the TIM-barrel template), beta-sandwich, and alpha-helix circular array. This diversity in cellulase fold structure must be taken into account when considering the transfer and application of design strate gies between various cellulases.