Enzymes are proteins that accelerate, or catalyze, chemical reactions. In these reactions, the molecules at the beginning of the process are called substrates and the enzyme converts these into different molecules: the products. Almost all processes in the cell need enzymes in order to occur at significant rates. Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell.

(source: Wikipedia)

Sigma-Aldrich		Enzyme explorer ressource: Search by name, application, specificity or EC number. The Enzyme Explorer also features detailed sites on metabolic pathways, protein kinases, protease specificity and inhibition, and glycoprotein analysis as well as new cell signaling, analytical and diagnostic enzymes and reagents.
Biotium		chromogenic, fluorogenic and bioluminescent enzyme substrates