Q.Draw the structure of the amino acid, alanine.

Alanine is one of the twenty amino acids that occur in proteins, and like every amino acid it can be pictured as a substituted methane — a single central carbon (the alpha-carbon) holding four different groups.

• At the centre sits the alpha-carbon.
• To it are attached, on separate valency positions, a hydrogen atom, a carboxyl group (written COOH) and an amino group (written NH2).
• The fourth position carries the R group, which for alanine is a methyl group (CH3). …

Amino acids are the building units of proteins, and all of them share the same simple plan, so once you can picture one you can picture them all. The trick is to think of the molecule as a substituted methane — a single central carbon atom that has four valency positions to fill.

That central carbon is called the alpha-carbon, and in every amino acid three of its four positions are occupied by the same three things:

• a hydrogen atom,
• a carboxyl group, the acidic part, written COOH, and
• an amino group, the basic part, written NH2.

Because both the amino group and the carboxyl group hang off the very same carbon, amino acids are called alpha-amino acids.

The fourth position is the interesting one. It is occupied by a variable side chain called the R group, and the identity of this R group is exactly what tells one amino acid apart from another. If the R group is just a hydrogen you get glycine; if it is a hydroxy-methyl group you get serine; and if it is a methyl group (CH3), you get alanine.

To actually set the structure down on paper, imagine the alpha-carbon in the centre and add the four groups around it:

• a hydrogen atom placed above the central carbon, …

Method: Drawing a Structure from the General Amino Acid Template

Rather than memorising each amino acid's structure separately, the efficient approach is to memorise ONE general template — the substituted-methane picture of an alpha-amino acid — and then simply plug in the correct side chain for whichever amino acid is asked about.

The template: a single central carbon (the alpha-carbon) with four attachment points. Three of those four positions are always the same for every amino acid — a hydrogen atom, an amino group (NH2), and a carboxyl group (COOH). The fourth position is the variable R group, and it is this one substitution that turns the generic template into a specific, named amino acid. For alanine specifically, the R group is a methyl group (CH3) — that one fact is the only thing you need to add on top of the template to answer this question correctly. …