What is pH?
pH is a measure of the hydrogen ion concentration, with values from 1 to 14. In this range, pH 7 is neutral, below pH7 is referred to as an acid, above pH7 is an alkali (or basic).
The higher the concentration of H+, the lower the pH value. This means that, in a solution, acids contain a higher concentration of H+.
In chemical terms, an acid is defined as a proton donor because hydrogen ions are also known as protons.

pH and bonds
A hydrogen ion carries a positive charge, so it will be attracted towards negatively charged ions, molecules or parts of molecules. As like charges repel, hydrogen ions will be repelled by positively charged ions, molecules or parts of molecules.
Large numbers of hydrogen bonds and ionic bonds are responsible for holding the tertiary structure of an enyme protein in place. This, in turn, ensures that the active site of the enzyme is held in the right shape. These bonds are due to the attraction between oppositely charged groups on the amino acids that make up the enzyme protein.
Because of their charge, hydrogen ions can interfere with the hydrogen bonds and ionic bonds holding the tertiary structure in place. This means that increasing or decresaing the concentration of the hydrogen ions around an enzyme can alter the tertiary structure of the enzyme molecule. Changes in pH can cause changes to the shape of the active site, and so change the rate of an enzyme-controlled reaction.

pH and active sites
The induced-fit hypothesis states that an important part of catalysis in the active site relies on charged groups on the R-groups of the amino acids that make up the active site. Increasing the concentration of hydrogen ions will alter the charges around the active site, as more hydrogen ions are attracted towards any negatively charged groups in active site.

Optimum pH varies between enzymes
All enzymes have their own optimum pH. This is the pH at which the rate of reaction is highest. For many enzymes the optimum pH is around neutral.
At the optimum pH, the concentration of the hydrogen ions in solution gives the tertiary structure of enzyme the best overall shape. This shape holds the active site in the shape that best fits the substrate.
Enzymes usually work in a fairly narrow pH range. Changes to pH, altering the concentration of hydrogen ions even very slightly, result in a fall in the reaction rate because the shape of the enzyme molecule is disrupted and so the shape of the active site is changed.
Measuring the effect of pH on enzyme action usually involves carrying out enzyme-controlled reactions at different pH values using buffer solutions. The production of a product, or the disappearance of a substance, can be measured in a varitey of ways.

pH and location
The wide range of environments in which enzymes have to work includes a range of pH levels. You can see a clear example of enzymes operating in different pH conditions in the human digestive system. In the stomach, the protein-digesting enzme pepsin works well. Pepsin has an optimum pH of around pH2. This is ideal for working in the stomach, which contains hydrochloric acid, giving it a pH of arounf 2.
Partially digested food leaving the stomach moves into the small intestine. Intestinal secretions neutralise the the food, so it can be digested. The protein-digesting enzyme trypsin works in small intestine. It has an optimum pH of around 7, ideal for the environment of the small intestine.

	Created by siobhan.quirk over 11 years ago

Undergraduate Biology (Biological Molecules) Note on Enzymes at Work - pH Effects, created by siobhan.quirk on 30/05/2013.