sentences:  A key feature of enzyme catalysis over many non-biological catalysis, is that both acid and base catalysis can be combined in the same reaction.In many abiotic systems, acids (large [H+]) or bases ( large concentration H+ sinks, or species with electron pairs) can increase the rate of the reaction; but of course the environment can only have one overall pH (measure of acidity or basicity (alkalinity)).However since enzymes are large molecules, they can position both acid groups and basic groups in their active site to interact with their substrates, and employ both modes independent of the bulk pH.