Question

isoelectric point of pepsin pepsin is the name given to a mix of several digestive enzymes secreted (as larger precursor proteins) by glands that line the stomach. these glands also secrete hydrochloric acid, which dissolves the particulate matter in food, allowing pepsin to enzymatically cleaved individual protein molecules. the resulting mixture of food, hcl, and digestive enzymes is known as chyme and has a ph near 1.5. what pl would you predict for the pepsin proteins? what functional groups must be present to confer this pl on pepsin? which amino acids in the proteins would contribute such groups?

Answer 1

The isoelectric point (pI) of a protein is the pH at which the protein carries no net electrical charge. For proteins like pepsin, which contain various amino acids with different acidic and basic side chains, determining the pI involves identifying the pH at which the positive and negative charges on the amino acids balance.

Given that the pH of the chyme (mixture of food, HCl, and digestive enzymes) is around 1.5, this indicates an acidic environment in the stomach.

For pepsin to have a pI lower than the chyme pH of 1.5, it must have a net positive charge at this pH. This means that the functional groups contributing to the pI of pepsin would be mostly protonated amino groups (NH₃⁺) from amino acids' side chains.

The amino acids that contribute ionizable amino groups and can influence the pI of proteins are:

1. Lysine (Lys): Lysine has an amino group that can be protonated and carry a positive charge at low pH. It has a pKa value around 10.5, which means that at low pH, the amino group is likely to be protonated.

2. Arginine (Arg): Arginine also has an amino group that can be protonated at low pH. It has a pKa value around 12.5, making it more likely to be protonated at low pH.