Figure 2.21 Using isozymes as adaptations to changing water temperatures. In winter, the rainbow trout produces a form of the enzyme acetylcholinesterase that has a high substrate affinity at 0°C to 10°C, but a lower substrate affinity at warmer temperatures. In summer, the trout produces a different form of the enzyme, which has a high substrate affinity at 10°C to 20°C, but a lower substrate affinity at colder temperatures.