Enzyme Pair Chooses Targets Based on Protein's Physical Surroundings

Kuroda and colleagues, writing in the Journal of Biochemistry in 1979, investigated why two closely related cellular enzymes act differently depending on how their target protein is positioned. The enzymes, protein kinase A and protein kinase G, both attach phosphate groups to proteins, a chemical tag that switches many cell processes on or off.

Working with purified enzymes and proteins in laboratory assays, the researchers found that protein kinase A worked best on histone, a protein that helps package DNA, when that histone was free-floating and unattached. Protein kinase G, in contrast, strongly favored histone that was bound to polydeoxyribonucleotide, a chain of DNA building blocks. When the researchers instead offered a different, freely soluble target called phosphorylase kinase, protein kinase A consistently outperformed protein kinase G.

This is a test-tube biochemical study, not an experiment in living cells or animals. The authors propose that the physical arrangement between an enzyme and its target, which they call topography, may better explain these preferences than the chemical makeup of the substrate alone. The abstract gives no detail on the source of the enzymes or the polydeoxyribonucleotide used, and it draws no conclusions about effects in living organisms.