Protein property – What is the approximate isoelectric pH (pI) of cytochrome c as commonly reported for the classic horse heart protein?

Introduction / Context:
The isoelectric point (pI) of a protein is the pH at which its net charge is zero. Cytochrome c, a small heme protein of the electron transport chain, has a relatively high pI because it is rich in basic residues. Knowing approximate pI values helps in designing electrophoresis and chromatography protocols, especially isoelectric focusing and ion-exchange separations.

Given Data / Assumptions:

• Reference is the well-studied horse heart cytochrome c used as an analytical standard.
• “Approximate” means recognizing the basic nature and order of magnitude rather than an exact universal constant.
• Buffers and post-translational modifications can shift observed pI slightly.

Concept / Approach:
Cytochrome c is lysine-rich and binds to negatively charged partners on the mitochondrial inner membrane. Its basic composition yields a pI around 10. The reported values in textbooks typically range near 10 to a little above 10 depending on measurement method, making 10.05 a reasonable representative option among the choices provided.

Step-by-Step Solution:

Verification / Alternative check:
Empirical isoelectric focusing gels and database entries list cytochrome c pI around 10–10.5 for common species, supporting the selection of 10.05 as an approximate value.

Why Other Options Are Wrong:

• 7.5 and 8.5: too acidic for cytochrome c’s known basicity.
• 11.05: higher than most reported values for typical preparations.
• 9.0: still lower than the widely cited basic pI near 10.

Common Pitfalls:
Assuming one exact pI for all species and conditions; minor sequence or modification differences can shift pI modestly, but not enough to move it down near neutrality for cytochrome c.

Final Answer:
10.05