MCAT Biochemistry Review

Chapter 2: Enzymes

2.4 Effects of Local Conditions on Enzyme Activity

The activity of an enzyme is heavily influenced by its environment; in particular, temperature, acidity or alkalinity (pH), and high salinity have significant effects on the ability of an enzyme to carry out its function. Note that the terms enzyme activityenzyme velocity, and enzyme rate are all used synonymously on the MCAT.


Enzyme-catalyzed reactions tend to double in velocity for every 10°C increase in temperature until the optimum temperature is reached; for the human body, this is 37°C (98.6°F or 310 K). After this, activity falls off sharply, as the enzyme will denature at higher temperatures, as shown in Figure 2.7. Some enzymes that are overheated may regain their function if cooled. A real-life example of temperature dependence occurs in Siamese cats. Siamese cats are dark on their faces, ears, tails, and feet but white elsewhere. Why? The enzyme responsible for pigmentation,tyrosinase, is mutated in Siamese cats. It is ineffective at body temperature but at cooler temperatures becomes active. Thus, only the tail, feet, ears, and face (cooled by air passing through the nose and mouth) have an active form of the enzyme and are dark.

Figure 2.7. Effects of Temperature and pH on the Rate of Enzyme Action


Most enzymes also depend on pH in order to function properly, not only because pH affects the ionization of the active site, but also because changes in pH can lead to denaturation of the enzyme. For enzymes that circulate and function in human blood, this optimal pH is 7.4, as shown in Figure 2.7. A pH < 7.35 in human blood is termed acidemia. Even though it's more basic than chemically neutral 7.0, it is more acidic than physiologically neutral 7.4. Where might exceptions to this pH 7.4 occur? Both are in our digestive tract. Pepsin, which works in the stomach, has maximal activity around pH 2, whereas pancreatic enzymes, which work in the small intestine, work best around pH 8.5.


The pH levels in the stomach and intestine, and their effects on these gastric and pancreatic enzymes, are covered in the Chapter 9 of MCAT Biology Review.


While the effect of salinity or osmolarity is not generally of physiologic significance, altering the concentration of salt can change enzyme activity in vitro. Increasing levels of salt can disrupt hydrogen and ionic bonds, causing a partial change in the conformation of the enzyme, and in some cases causing denaturation.

MCAT Concept Check 2.4:

Before you move on, assess your understanding of the material with these questions.

1.    What are the effects of temperature, pH, and salinity on the function of enzymes?

·        Temperature:

·        pH:

·        Salinity:

2.    What is the ideal temperature for most enzymes in the body? The ideal pH?

·        Ideal temperature:  °C =  °F =  K

·        Ideal pH (most enzymes): 

·        Ideal pH (gastric enzymes): 

·        Ideal pH (pancreatic enzymes):