Figure 8.1 The effect of heat on enzyme activity. Enzyme activity increases with temperature until the enzyme is denatured. The enzyme tyrosinase, which is part of the pathway that synthesizes the pigment that results in dark fur, has a low tolerance for heat in Siamese cats. It is inactive at normal body temperatures but functional at slightly lower temperatures. The extremities of a Siamese cat are cool enough for tyrosinase to gain function and produce pigment.