Question

For each of the following, name an amino acid in which the \(\mathrm{R}\) group contains it: a hydroxyl group, a sulfur atom, a second chiral carbon atom, an amino group, an amide group, an acid group, an aromatic ring, and a branched side chain.

Short answer

Serine, cysteine, threonine, lysine, asparagine, aspartic acid, phenylalanine, valine.

Step-by-step solution

Identify an amino acid with a hydroxyl group

The amino acid serine (Ser, S) has a hydroxyl group (-OH) in its \(\text{R}\) group.

Identify an amino acid with a sulfur atom

The amino acid cysteine (Cys, C) has a sulfur atom (S) in its \(\text{R}\) group.

Identify an amino acid with a second chiral carbon atom

The amino acid threonine (Thr, T) has a second chiral carbon atom in its \(\text{R}\) group.

Identify an amino acid with an amino group

The amino acid lysine (Lys, K) has an additional amino group (-NH2) in its \(\text{R}\) group.

Identify an amino acid with an amide group

The amino acid asparagine (Asn, N) has an amide group (-CONH2) in its \(\text{R}\) group.

Identify an amino acid with an acid group

The amino acid aspartic acid (Asp, D) has an acid group (-COOH) in its \(\text{R}\) group.

Identify an amino acid with an aromatic ring

The amino acid phenylalanine (Phe, F) has an aromatic ring in its \(\text{R}\) group.

Identify an amino acid with a branched side chain

The amino acid valine (Val, V) has a branched side chain in its \(\text{R}\) group.

Key concepts

hydroxyl group

A hydroxyl group is a functional group consisting of an oxygen atom bonded to a hydrogen atom (-OH). It is polar and can form hydrogen bonds, making compounds that contain it soluble in water. In amino acids, the hydroxyl group is found in serine (Ser, S). This makes serine more reactive and allows it to play a significant role in biochemical reactions, such as phosphorylation.

sulfur atom

A sulfur atom is a chemical element found in some amino acids. It adds hydrophobic (water-repelling) properties and can form disulfide bonds, which are crucial for the structural stability of proteins. Cysteine (Cys, C) is an amino acid that contains a sulfur atom. The sulfur allows cysteine to form disulfide bridges with other cysteine molecules, which helps stabilize protein structures like those in hair and feathers.

chiral carbon

A chiral carbon is a carbon atom that is attached to four different groups, making it a center of chirality and resulting in two non-superimposable mirror image forms called enantiomers. Threonine (Thr, T) has a second chiral carbon in its R group. This additional chiral center adds to the complexity of protein folding and function because it can affect the 3D orientation of the amino acid.

amino group

An amino group consists of a nitrogen atom bonded to two hydrogen atoms (-NH2). It is basic and can accept protons, making it crucial for forming peptide bonds in proteins. Lysine (Lys, K) contains an additional amino group in its R group. This extra amino group makes lysine positively charged at physiological pH, which can influence protein structure and interactions.

amide group

An amide group consists of a carbonyl group (C=O) linked to a nitrogen atom (-CONH2). It is derived from carboxylic acids and amines. Asparagine (Asn, N) has an amide group in its R group. The presence of the amide group allows asparagine to form multiple hydrogen bonds, enhancing protein stability and solubility in water.

acid group

An acid group commonly refers to a carboxyl group (-COOH), which can release protons, making the molecule acidic. Aspartic acid (Asp, D) contains an acid group in its R group. This group can participate in acid-base reactions and is often involved in enzyme active sites, contributing to the regulation of pH and enzyme activity.

aromatic ring

An aromatic ring is a planar ring of atoms with delocalized π-electrons that follow Huckel's rule (4n + 2 π-electrons). This structure is stable and often participates in stacking interactions. Phenylalanine (Phe, F) has an aromatic ring in its R group. The aromatic ring adds hydrophobic character to the amino acid and can engage in interactions that stabilize protein structures.

branched side chain

A branched side chain is when the side chain of an amino acid has a branching point. This creates more complex and less flexible structures, often resulting in hydrophobic characteristics. Valine (Val, V) has a branched side chain in its R group. This branching contributes to the hydrophobic core of proteins, playing a role in protein folding and stability.