ABSTRACT
Enzyme velocity The rate of an enzyme-catalyzed reaction is often called its velocity. Enzyme velocities are normally reported as values at time zero (initial velocity, symbol V
since the rate is fastest at the point where no product is yet present. This is because the substrate concentration is greatest before any substrate has been transformed to product, because enzymes may be subject to feedback inhibition by their own products and/ or because with a reversible reaction the products will fuel the reverse reaction. Experimentally V0 is measured before more than approximately 10% of the substrate has been converted to product in order to minimize such complicating factors. A typical plot of product formed against time for an enzyme-catalyzed reaction shows an initial period of rapid product formation, which gives the linear portion of the plot (Figure 1). This is followed by a slowing down of the enzyme rate as substrate is used up and/or as the enzyme loses activity. V0 is obtained by drawing a straight line through the linear part of the curve, starting at the zero time-point (Figure 1). The slope of this straight line is equal to V
describes these observations and predicts a hyperbolic curve of V
sum of the rates of breakdown of the enzyme-substrate complex over its rate of formation, and is a measure of the affinity of an enzyme for its substrate.
