The isoelectric point (pI) of human chorionic gonadotropin (hCG) is notably acidic, with some variants reaching a pI as low as 3.1.
Understanding hCG's Acidic pI
hCG's exceptionally low pI is primarily attributed to its high level of sialylation. Sialic acid residues are negatively charged, and hCG is a highly sialylated glycoprotein, possessing up to 15 sialic acid residues per molecule. This abundance of negative charges significantly lowers its isoelectric point.
Factors Affecting hCG pI:
- Sialylation: The degree of sialylation is the most significant factor influencing the pI of hCG. Variations in sialylation patterns can lead to different hCG isoforms with slightly different pI values.
- Glycosylation: Other glycosylation patterns, besides sialylation, also contribute to the overall charge and, therefore, the pI of hCG.
- Amino Acid Composition: While glycosylation plays a dominant role, the amino acid composition of the hCG protein also contributes to its overall charge and pI.
Significance of pI
The pI of a protein, including hCG, is the pH at which the protein has no net electrical charge. This property is important in various biochemical techniques such as isoelectric focusing and ion exchange chromatography, which are used for separating and purifying proteins based on their charge.
