Protein engineering

Protein engineering changes a protein to give it new or better properties. Scientists find the key amino acids responsible for a property, and change the codon for one of them by a point mutation in the gene: site-directed mutagenesis. Some examples:

A tougher detergent enzyme. Subtilisin, a protein-digesting enzyme used in detergents, works through three key amino acids (Ser221, His64, Asp32). But bleach in the wash oxidises its methionine at position 222, next to the active site, and weakens it. Changing Met222 to alanine made subtilisin stable in bleach.

Easy purification. A 6-His-tag (six histidines) added to the end of a protein binds firmly to nickel, zinc or cobalt at mildly alkaline pH. So when a cell extract is poured through a nickel column, the tagged protein sticks while all other proteins wash through; it is then released (eluted) pure.

Seeing proteins in cells. The gene for GFP (green fluorescent protein), from the jellyfish Aequorea victoria, can be fused to the gene for any protein. The fused protein glows green under UV light, showing exactly where it goes in a living cell: the nucleus, the mitochondria or the cytoplasm.

Targeted cancer drugs. An immunotoxin joins the binding part of an antibody (an scFv: the variable regions VH and VL joined by a short linker) to a toxin such as Pseudomonas exotoxin A, diphtheria toxin or ricin. The antibody part guides the toxin to cancer cells, killing them selectively.

Humanised antibodies. Mouse antibodies are recognised as foreign by the human immune system. So the antigen-binding part of a mouse antibody gene is moved into a human antibody gene. The result works like the mouse antibody, but the patient's body accepts it.