Question

How are proteins able to provide a buffering action?

Short answer

Proteins provide a buffering action through the ionizable side chains of acidic (Asp and Glu) and basic (Arg, His, Lys) amino acid residues, as well as the N-terminal (-NH2) and C-terminal (-COOH) groups of the protein. These groups can donate or accept protons to maintain a relatively constant pH when acids or bases are added to the solution.

Step-by-step solution

1. Understand protein structure

Proteins are made up of amino acids connected by peptide bonds. Each amino acid has a central carbon atom (C-alpha) bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a side chain (R group) that is unique to each of the 20 commonly occurring amino acids.

2. Understand amino acid side chains

The side chains (R groups) of amino acids can be classified into different categories based on their chemical properties. Some of these side chains have acidic or basic properties, which can participate in hydrogen ion interactions and contribute to the buffering capacity of proteins.

3. Identify acidic and basic amino acid residues

There are two kinds of amino acid side chains that can contribute to the buffering action of proteins: acidic and basic side chains. Acidic side chains include aspartic acid (Asp) and glutamic acid (Glu), which have carboxyl groups (-COOH) in their side chains. Basic side chains include arginine (Arg), histidine (His), and lysine (Lys), which have amino groups (-NH2) in their side chains.

4. Explain the buffering action of acidic and basic side chains

The buffering action of proteins occurs due to the ionizable side chains of acidic and basic amino acid residues. When an acid is added to a protein solution, the acidic side chains of Asp and Glu can act as proton acceptors, while the basic side chains of Arg, His, and Lys can act as proton donors. This counteracts the excess protons from the acid, minimizing the change in pH. Conversely, when a base is added to a protein solution, the basic side chains can accept protons from the solution, and acidic side chains can donate protons to the solution, preventing a substantial pH change.

5. Consider the buffering action of N-terminus and C-terminus

In addition to the ionizable side chains of amino acid residues, the N-terminal amino group (-NH2) and the C-terminal carboxyl group (-COOH) of a protein can also participate in buffering action. When the pH of the solution is close to the pKa of N-terminal or C-terminal groups, these groups can also act as proton acceptors or donors, providing additional buffering capacity. In conclusion, proteins provide a buffering action through the ionizable side chains of acidic and basic amino acid residues, as well as the N-terminal and C-terminal groups of the protein. These groups can donate or accept protons to maintain a relatively constant pH when acids or bases are added to the solution.