The efficiency of an enzyme in catalyzing a reaction is due to its capacity_____________.

A catalyst is a substance, which increases the rate of reaction without itself being consumed at the end of the reaction, and the phenomenon is called catalysis. There are some catalysts which decrease the rate of reaction and such catalysts are called negative catalyst. Obviously, the catalyst accelerating the rate will be positive catalyst. However, the term positive is seldom used and catalyst itself implies positive catalyst. 
Enzymes acts as catalyst for biological process and lower activation energy and it increases efficiency.