What is the optimum temperature for the enzyme catalase in liver?

Answer 1

What we expect in an enzyme reaction is that it goes faster and faster as temperature is increased until a temperature is reached at which the enzyme is denatured (its shape is changed and therefore its activity is destroyed) before much reaction can be measured. So, as we increase the temperature, two things are happening. The rate of reaction is getting faster - all chemical reactions go faster at higher temperatures. But also, the shape of the enzyme molecule gets changed so that it is less and less efficient as a catalyst . The value of the optimum temperature is therefore not a fixed number but depends upon the way in which the experiment was carried out. The shorter the measurement time the higher the apparent optimum temperature will be. If you were able to measure the rate during the first second at the high temperature, you might measure a very fast rate of reaction before the enzyme had time to be denatured. However, if you measure the rate over a period of ten mins. there may have been no effect of the enzyme at all in the final nine mins. and the measured rate of reaction would be very slow indeed - almost the same as if there had been no enzyme there at all. Therefore, you cannot quote a figure for the optimum without explaining the whole experiment in which the measurements were made.

A second question is "How much does catalase respond to an increase in temperature up to the point at which it is denatured?". For most enzyme catalysed reactions, there is approximately a doubling of the rate of reaction for every 10ºC rise. This is called a of two and most enzymes have a between 1.5 and 2.5. The strange thing about catalase is that it has a of less than 1.2 (between 0 and 40ºC). This means that temperature has hardly any effect on the rate of reaction with catalase. In other words, there is hardly any increase in the rate of reaction as the temperature increases. It turns out that the rate at which catalase can work is limited by the rate at which the substrate (peroxide) can diffuse into the "active site" of the enzyme and this diffusion is a physical rather than a chemical process (and is therefore much less affected by temperature). The paper by R. F. Beers and I. W. Sizer (1952) J. Biol. Chem. 195, 133 which discusses the assay and mentions the essential temperature-independence can be freely downloaded from the internet http://intl.jbc.org/

Answer 2

40 oC but that isn't very precise

Answer 3

The optimum temperature is 40ºC