Enzyme-substrate complex ($ES$)

An enzyme-substrate complex is the bound association of an enzyme with a substrate before or during catalysis, commonly written $ES$. In the scheme $E+SightleftharpoonsESoE+P$, binding, unbinding and product formation are distinct steps. The amount of $ES$ need not increase indefinitely with substrate concentration because the supply of enzyme is finite. A bound complex also need not be the transition state or proceed to product on every encounter. ^[ref]