The optimum temperature is about 40oC because the average human body temperature
However, a common misconception on enzyme action is that the optimum temperature
for enzyme activity is 37oC, which is the human body temperature.
If we define optimum temperature as the temperature at which the enzyme reaches
its maximum rate, this statement is certainly not true for most enzymes, even
for human enzymes. In man and many other organisms, most enzymes have an optimum
temperature of 45-50oC, and some can function well above 37oC.
For example, the optimum temperature of human salivary amylase, based on many
students' work, lies between 60oC to 70oC. A few enzymes
are heat-resistant, such as papain, a protease from papaya fruit, and a similar
enzyme from pineapple. They are used as meat tenderizers because their proteolytic
action can continue at cooking temperatures as high as 70-80oC, although
at the temperature of boiling they are rapidly denatured.
If most enzymes in our body
have an optimum temperature of 45-50oC, why is it that we cannot
survive when our body temperature rises for just a few degrees above 37oC?
A likely reason is that certain key enzymes in the main metabolic pathways
of the body have a relatively low optimum temperature, such as around
40oC. As the body temperature goes beyond 40oC,
some important metabolic pathways of the body would be impaired although
most other enzymes continue to work properly. This therefore imposes a
limit to the highest temperature that the body can tolerate.
If the temperature is reduced
to near or below freezing point, enzyme activity will become very low
or even stop. The enzyme is said to be inactivated, but not denatured.
This is a point that many students may easily get confused with. The reduced
activity is due to a much lower speed of movement of the substrate molecules,
leading to a low frequency of collision, and not due to the loss of natural
configuration of the enzyme molecule. The enzyme will regain its catalytic
power when it is restored to a higher temperature.
Techniques of deep-freezing
are commonly used to preserve food for extensive periods. The very low
temperature not only prevents the growth and multiplication of micro-organisms,
but also inactivates their digestive enzymes and the natural enzymes present
in the food itself, such as meat and vegetables, thus making it impossible
for them to decompose the food.
The effects of temperature
on enzyme activity can be summarized as follows: The optimum temperature
for an enzyme is the temperature at which the enzyme reaches its maximum
rate. As the surrounding temperature deviates from the optimum temperature,
enzyme activity drops accordingly, and at extremely low and high temperatures
(relative to the optimum temperature), the enzyme becomes inactivated.
However, the mechanism of enzyme inactivation is different. At high temperatures,
the enzyme is inactivated due to denaturation of the enzyme molecule while
at low temperatures, the enzyme becomes inactivated because of a drop
in the kinetic energy of the substrate molecules.
The two different mechanisms
are reflected by the shape of the curve showing the effects of temperature
on enzyme activity (Fig. 1). The slope of the curve left to the optimum
temperature has a temperature quotient of about two (Q10 = 2); with a
rise in temperature within this range, there is a steady increase in the
reaction rate due to increased speed of movement of the substrate molecules.
On the other hand, with a rise of temperature above the optimum value,
enzyme activity drops drastically due to rapid denaturation of the enzyme
Figure 1 : Effect of temperature on enzyme activity