"V"max is only one of several important kinetic parameters. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the Michaelis–Menten constant ("K"m), which is the substrate concentration required for an enzyme to reach one-half its maximum reaction rate; generally, each enzyme has a characteristic "K"M for a given substrate. Another useful constant is "k"cat, also called the "turnover number", which is the number of substrate molecules handled by one active site per second.