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#protein pI

3 public questions tagged with this topic.

Electrophoresis

Which protein modification slows SDS-PAGE migration but doesn't change pI?

SDS-PAGE migration primarily reflects polypeptide length due to uniform SDS charge coating, overwhelming intrinsic charge. Post-translational modifications that add neutral mass retard mobility without altering isoelectric point. N-linked and O-linked glycosylation attaches bulky hydrophilic carbohydrate chains lacking proportional SDS binding and charged groups, increasing hydrodynamic size and apparent molecular weight substantially while pI remains largely unchanged because glycans are mostly neutral. This produces slower, often smeared migration. Phosphorylation, ubiquitination and acetylation add or remove charges, influencing both pI and mobility, unlike neutral glycosylation which mainly increases mass.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Electrophoresis

What is the pI of a protein that does not migrate at pH 7?

Isoelectric point defines pH at which a protein carries zero net charge because numbers of protonated basic residues and deprotonated acidic residues are balanced. When buffer pH equals pI, electrostatic attraction toward anode or cathode ceases, resulting in no net electrophoretic migration and stationary focusing. At pH 7, a protein that shows no movement must possess balanced charges at that pH, indicating its isoelectric point is approximately 7. Proteins with pI values of 5, 6 or 8 would carry net negative or positive charge at pH 7 and would migrate toward anode or cathode respectively under electric field.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Protein

The isoelectric point (pI) of a protein is the pH at which:

The protein has no net charge is the scientifically accurate answer to this question. Within the study of Protein Solubility, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of The protein has no net charge directly address what is being asked. Among the other options, The protein carries a net positive charge, The protein carries a net negative charge, and The protein is most soluble do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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