What Is the Structure of an Amino Acid?

An amino acid has a central alpha carbon atom bonded to four groups: an amino group, a carboxyl group, a hydrogen atom and a variable R group. This generalized structure is required at both SL and HL in IB Biology subtopic B1.2 Proteins.

The Generalized Structure

The structure can be represented as:

$\text{H}_2\text{N}-\text{CH}(\text{R})-\text{COOH}$

The central carbon is called the alpha carbon because it is directly adjacent to the carboxyl carbon.

There are 20 amino acids coded for by the genetic code. They share this generalized structure but have different R groups, allowing amino acids to vary in properties and enabling proteins to have diverse forms and functions.

During a condensation reaction, the carboxyl group of one amino acid reacts with the amino group of another. Water is released and a peptide bond forms, producing a dipeptide; repeated condensation produces a polypeptide.

A common misconception is that the R group always means one carbon-containing group. In fact, “R” is a placeholder for any variable side chain, whose structure differs between amino acids.

Exam Technique

For a “draw” question, show the alpha carbon with four separate attachments: $-\text{NH}_2$, $-\text{COOH}$, $-\text{H}$ and $-\text{R}$. Do not omit the hydrogen or attach the R group to the wrong atom. For an “explain” question, link variation in R groups to differences in amino acid properties and therefore protein form and function.