Amino Acids & Proteins

What are amino acids?

Amino acids are the building blocks of proteins which contain an amino group, a carboxylic group and a unique ``-R'' group. This specific arrangement about a carbon creates an $\alpha$-amino acid:

Figure 53.1: An amino acid.

Which amino acid has the simplest $R-$ group?

Glycine, whose $R-$ group is an $H-$. This makes glycine achiral and not optically active, while the 20 other amino acid $R-$ groups get increasingly more complex and chiral.

Figure 53.2: Glycine.

What is the absolute configuration of an amino acids?

$L$ configuration:

Figure 53.3: $L$-configuration of amino acids: $L$-proline.

Note: There are some D-configured amino acids that are found in bacteria, but the overwhelming majority of amino acids are L-configured.

What are two ways in which amino acids can be drawn?

Figure 53.4: Amino acid projections: The Fischer projections have the amino group on the left, and the equivalent wedge-dashed-wedge line projection is depicted on the right.

What makes amino acids bipolar ions?

The basic amino group and the acidic carboxyl acid group. Based on the $pH$ of the solution, the degree of protonation can vary:

Figure 53.5: Amino acids have a basic amino group and an acidic carboxylic acid group. Additionally, the $-R$ group can also have acidic or basic properties.

What links two amino acids and what is released when two amino acids polymerize?

A peptide linkage attaches two amino acids. When polymerization occurs between two amino acids to form a dipeptide, H$_{2}$O is released:

Figure 53.6: Peptide linkage (within the shaded region).

Note: Therefore, hydrolysis of a peptide linkage results in the addition of H$_{2}$O and cleavage of the bond.