ThrB

thrB is a bacterial gene that encodes the enzyme homoserine kinase, a key player in the biosynthesis of threonine within the aspartate family pathway. The encoded protein catalyzes the ATP-dependent phosphorylation of homoserine to O-phosphohomoserine, the next intermediate before threonine is formed.

In many bacteria, including Escherichia coli, thrB is part of a short operon with thrA and thrC. thrA encodes a bifunctional aspartokinase I/homoserine dehydrogenase I, thrB encodes homoserine kinase, and thrC encodes threonine synthase. The overall sequence of reactions converts aspartate into threonine, with homoserine kinase performing the phosphorylation step that commits homoserine toward threonine synthesis.

Biochemically, homoserine kinase belongs to the GHMP kinase superfamily. The enzyme is typically a soluble, cytosolic protein of about 300 amino acids and requires divalent metal ions such as Mg2+ for activity. Structural studies across species show a compact, mostly alpha-helical fold with conserved motifs characteristic of GHMP kinases, supporting a mechanism in which homoserine and ATP are positioned for phosphorylation.

Regulation and distribution vary among organisms. thrB is generally co-regulated with thrA and thrC at the operon level, and threonine availability can influence transcriptional regulation in some species. Homoserine kinase homologs are widely distributed in bacteria, archaea, and plastids of plants, reflecting the conserved need to synthesize threonine endogenously in organisms that rely on the aspartate pathway.