<unk> s are biomolecules that catalyze (i.e. increase the rates of) chemical reactions. Almost all<unk> s are proteins. In<unk> tic reactions, the molecules at the beginning of the process are called substrates, and the<unk>  converts them into different molecules, the products. Almost all processes in a biological cell need<unk> s in order to occur at significant rates. Since<unk> s are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of<unk> s made in a cell determines which metabolic pathways occur in that cell.
Like all catalysts,<unk> s work by lowering the activation energy (Ea or ΔG‡) for a reaction, thus dramatically increasing the rate of the reaction. Most<unk>  reaction rates are millions of times faster than those of comparable uncatalyzed reactions. As with all catalysts,<unk> s are not consumed by the reactions they catalyze, nor do they alter the equilibrium of these reactions. However,<unk> s do differ from most other catalysts by being much more specific.<unk> s are known to catalyze about 4,000 biochemical reactions. A few RNA molecules called ribozymes catalyze reactions, with an important example being some parts of the ribosome. Synthetic molecules called artificial<unk> s also display<unk> -like catalysis.<unk>  activity can be affected by other molecules. Inhibitors are molecules that decrease<unk>  activity; activators are molecules that increase activity. Many drugs and poisons are<unk>  inhibitors. Activity is also affected by temperature, chemical environment (e.g. pH), and the concentration of substrate. Some<unk> s are used commercially, for example, in the synthesis of antibiotics. In addition, some household products use<unk> s to speed up biochemical reactions (e.g.,<unk> s in biological washing powders break down protein or fat stains on clothes;<unk> s in meat tenderizers break down proteins, making the meat easier to chew).
Enzyme
Antigen
Substrate 
Isozyme
Caspase
Receptor 
Hydrolase
Biomolecule