Answer:
The isoelectric point (pI) of cysteine is 5.02. This is the pH at which the overall charge of a cysteine molecule is zero. At this pH, the carboxyl group is ionized (-COO-) and the amino group is protonated (+NH3+). The side chain of cysteine is a sulfhydryl group (-SH), which is not ionized at physiological pH.
The pI of cysteine is important because it affects the way that cysteine residues interact with each other and with other amino acids in proteins. At pH values below the pI, cysteine residues will have a net negative charge. This can cause them to interact with positively charged amino acids, such as lysine and arginine. At pH values above the pI, cysteine residues will have a net positive charge. This can cause them to interact with negatively charged amino acids, such as aspartic acid and glutamic acid.
The pI of cysteine can also affect the stability of proteins. Proteins are typically folded into specific three-dimensional structures. The pI of a protein can affect the way that the protein folds, and can therefore affect its stability. For example, if the pI of a protein is at a pH that is outside of the physiological range, the protein may unfold and lose its function.
The pI of cysteine is also important for the function of proteins that contain cysteine residues. For example, the enzyme cysteine protease uses cysteine residues to cleave peptide bonds. The pI of the enzyme is important for the activity of the enzyme, because it affects the way that the cysteine residues interact with the peptide bonds that they are cleaving.
Overall, the pI of cysteine is an important property of the amino acid. It affects the way that cysteine residues interact with each other and with other amino acids in proteins. It can also affect the stability of proteins and the function of proteins that contain cysteine residues.
Step-by-step explanation: