The greatest buffering capacity at physiologic pH would be provided by a protein rich in which of the following amino acids?
**Core Concept:** Buffering capacity is the ability of a solution to resist changes in pH when an acid or base is added. In the context of a protein, the amino acids that contribute to this property are those with negative charges at physiological pH, which counteract the protonation of other amino acids or the medium.
**Why the Correct Answer is Right:** Serine is an amino acid with a hydroxyl (-OH) group in its side chain, which can be protonated (OH-) or deprotonated (-OH). At physiological pH, the hydroxyl group is deprotonated, making serine an acidic amino acid that can accept protons and thus contribute to buffering capacity.
**Why Each Wrong Option is Incorrect:**
A. Alanine: Alanine has a carboxyl (-COOH) group in its side chain, which is protonated at physiological pH (COOH^2+). This makes it a basic amino acid and does not contribute to buffering capacity.
B. Lysine: Lysine has an amino (-NH2) group in its side chain, which is protonated at physiological pH (NH3^+). This makes it a basic amino acid and does not contribute to buffering capacity.
C. Aspartic acid: Aspartic acid has a carboxyl (-COOH) group in its side chain, which is protonated at physiological pH (COOH^2+). This makes it a basic amino acid and does not contribute to buffering capacity.
D. Glutamic acid: Glutamic acid is similar to aspartic acid, having a carboxyl (-COOH) group in its side chain, which is protonated at physiological pH (COOH^2+). This makes it a basic amino acid and does not contribute to buffering capacity.
**Clinical Pearl:** Buffering capacity in biological systems is crucial for maintaining pH homeostasis, as it allows the body to absorb and release protons as needed to counteract changes in pH.
**Correct Answer:** Serine
**Why Each Wrong Option is Incorrect:**
Amino acids with acidic (-COOH) groups, such as aspartic acid and glutamic acid, are protonated at physiological pH (COOH^2+) and thus contribute to basicity rather than acidity, causing them to have no buffering capacity. Basic amino acids like alanine, lysine, and arginine do not contribute to buffering capacity due to their protonated side chains at physiological pH.