The rate of an enzyme, its activity, may be expressed in several ways, depending on the purity of the enzyme and one's knowledge of its characteristics, e.g. molecular mass and active sites per molecule. Optimal conditions of substrate, cofactor and salt concentrations and temperature should be selected to give the maximal rate. In classical enzyme kinetics, the rate of conversion of the enzyme-substrate complex to enzyme and product, characterized by the first-order rate constant, k_cat, is assumed to be the rate-limiting step, however, in some particularly fast enzymatic reactions, this is not the case. If the concentration of active sites in a solution is known, activity may be expressed as a turnover number: substrate molecules converted into product per enzyme active site per minute, in units of min^-1. If the number of active sites is unknown or the enzyme preparation is impure, activity can be expressed as μmol substrate converted per μg protein per second. A unit of enzyme activity is defined as the amount of an enzyme that will convert 1 μmol of substrate into product per minute at 30 degrees celsius under optimal conditions of substrate concentration, cofactor, etc. A katal is the amount of enzyme that will convert 1 mol of substrate per second.