Why enzyme activity is affected by pH

Why enzyme activity is affected by pH

 

A pH is the measure of the concentration of hydrogen ions, H+ with their values ranging from 1 -14. pH 7 is said to be neutral, any value below this is said to be acidic and values above are alkaline. A high concentration of H+ results in a low pH value, meaning acidic solutions contain large concentrations of H+ ions. These H ions can also be known as protons, which is why acid solutions can said to be proton donors.

H ions have a positive charge which means that they are attracted to the negative parts of certain molecules (or negative ions).

An enzymes tertiary structure is held in place by a number of ionic and hydrogen bonds. These bonds help to ensure that the active site of the enzyme is held in the right shape. These bonds occur because of the attraction between oppositly charged groups on the amino acids that make up the enzyme protein.

Because of the charge of the H+ ions, they are able to interfere with the hydrogen bonds and the ionic bonds that are holding the enzymes tertiary structurre in place. This means that increasing or decreasing the concentration of hydrogen ions in a soluition around an enzyme ( changing the pH of the solution) can alter the tertiary structure of the enzyme molecule. So changes in pH can also cause changes to the shape of the active site , so it is therefore able to change the rate of an enzyme controlled reaction (because the shape of the substrate is complementry to the shape of an active site and enzymes are specific; meaning that they are only able to catalyse a reaction involving only one type of substrate- so if the active site changes, enzyme substrate complexes would not be able to be formed and so the enzyme would not be able to catalyse that specific type of reaction; so it would change the rate of an enzyme controlled reaction).

The induced fit hypothesis (the binding of the substrate to the active site resulting in a change of shape in the enzyme so that it fits around the substrate more closly) states that an important part of catalysis in the active site relies on charged groups of on The R-groups of the amino acids that make up the active site. So, incresing the pH (the concentration of H ions) will alter the charges that are around the active site. This would happen because more H ions would be attracted towards any negativly charged groups that are in the active site (amino acids form part of the active sites and are negativly charged. These H+ions are therfore attracted to these groups and cluster around them. By doing this, it interferes with the binding of the substrate to the active site and it therefore changes the rate of activity of the enzyme – the rate of an enzyme controlled reaction).

 

Every enzyme has their own optimum pHs; the pH where the rate of the enzyme controlled reaction is at its highest. The majority of enzymes have an optimum pH of around 7, which is neutral. The optimum pH of an enzyme means that the concentration of H + ions in the solution is the right concentration to give the tertiary structure of the enzyme its best overall shape. This shape is responsible for holding the active site of the enzyme in the shape that best fits the substrate – meaning the shape which is complementry to that of the substrate (meaning reactions would be able to be catalysed).

 

All enzymes generally work in a fairly narrow pH range. A change in pH (which would alter the concentration of H+ ions), even a slight change in pH would mean that the reaction rate (of the enzyme catalysed reaction) would fall. This is because the shape of the enzyme molecule would have been disrupted and therfore the shape of the enzymes active site would have been changed (so it would therefore no longer be able to accommodate the subsrate and catalyse specific reactions – reactions involving only one type of substrate because the the active site has changed). However, changes in pH values would not actually denature an enzyme. The bonds would be disrupted due to the changes in pH but they are able to reform if the pH returns to its optimum level. The denaturisation of enzymes would only occur if any extreme changes of pH occurs away from its optimum pH.

 

Enzymes may also be affected by pHs in the natural environment, meaning that enzymes may have to work in a variety of different environments which may involve working at different pH levels. So enzyme activity may be affected by pH because it has to be (due to its environmental conditions).The human digestive system is a good example of this; pepsin is a protein digesting enzyme and works well in the stomach with an optimum pH of around 2. This is an ideal pH for working in the stomach, because the stomach contains HCLwhich is why the stomach has a pH of around2.


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