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An amino acid combines carboxylic acid and amine functionality in the same molecule; the name alone does not specify their separation. An alpha-amino acid has its amino nitrogen attached to the carbon immediately adjacent to the carboxyl carbon, the amino acid alpha carbon. For example, ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ is glycine, whereas ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ has an extra intervening carbon and is not an alpha-amino acid. These chemical formulas describe connectivity, not the predominant protonation state in water.

Most common proteinogenic amino acids share the skeleton ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}(R)-\mathrm{COOH}$, with $R$ the amino acid side chain. In glycine, $R$ is hydrogen; in alanine, $R=\mathrm{C}{\mathrm{H}}_3$. Proline is a useful exception to the displayed primary-amine skeleton: its amino nitrogen is part of a ring and has a second carbon substituent. The shared backbone permits assembly into polypeptides, while different amino acid side chains give the resulting proteins diverse chemistry. ^[ref]

In a chemical solution, protonation changes the electric charges without changing this backbone. Free glycine near neutral pH is predominantly ${\mathrm{H}}_3{\mathrm{N}}^{+}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CO}{\mathrm{O}}^{-}$, a zwitterion. Its positive and negative groups are joined within one molecule; zero net electric charge does not mean absence of local electric charges, nor does it mean the uncharged drawing ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ predominates. Glycine protonation explains how lowering or raising pH changes the balance. ^[ref]

Building a protein joins the carboxyl carbon of one amino acid to the amino nitrogen of another through a peptide bond, an amide linkage. The formal net condensation loses the elements of water; cellular translation uses activated substrates at a ribosome, rather than simply mixing free amino acids in water. Incorporated units are amino acid residues. The interior amide groups no longer have the same acid/base behavior as the two free terminal groups of glycine, so its free-molecule glycine titration cannot be copied independently at every chain position. ^[ref]

Chirality distinguishes most common proteinogenic amino acids from glycine. The other nineteen members of the conventional twenty have a stereocenter at their amino acid alpha carbon; ordinary translation incorporates their L forms. Glycine's two identical hydrogen substituents make it achiral. Thus being an amino acid, or even an alpha-amino acid, does not by itself imply chirality. ^[ref]

Enantiomers are a pair of nonsuperposable mirror-image molecules. The D and L configurations of alanine are an example; glycine has no such pair because its amino acid alpha carbon carries two identical hydrogen chemical atoms. ^[ref]

The isoelectric point is the pH at which a specified molecule's mean net electric charge is zero. Different protonation states can still carry nonzero electric charges; their contributions cancel in the average. For the two-step glycine speciation model, this gives $\mathrm{pI}=(\mathrm{p}{K}_{a1}+\mathrm{p}{K}_{a2})/2$. Additional ionizable amino acid side chains require their own chemical equilibrium calculations; averaging every pKa is not a general rule. ^[ref]

Below its isoelectric point, free glycine has positive mean electric charge and an electric field drives it toward the negative electrode; above its isoelectric point, the mean electric charge is negative and the direction reverses. At its isoelectric point the charge-driven contribution vanishes. Diffusion and bulk movement of the chemical solution can still move the sample. Gel electrophoresis applies this charge dependence to separation, although protein size, shape and interactions also affect migration. The pKa values of free glycine do not determine a protein's isoelectric point: its free N-terminus, C-terminus and ionizable amino acid side chains supply a different inventory. ^[ref]