Structure
Galactokinase is composed of two domains separated by a large cleft. The two regions are known as the N- and C-terminal domains, and the adenine ring of ATP binds in a hydrophobic pocket located at their interface. The N-terminal domain is marked by five strands of mixed beta-sheet and five alpha-helices, and the C-terminal domain is characterized by two layers of anti-parallel beta-sheets and six alpha-helices. Galactokinase does not belong to the sugar kinase family, but rather to a class of ATP-dependent enzymes known as the GHMP superfamily. GHMP is an abbreviation referring to its original members: galactokinase, homoserine kinase, mevalonate kinase, and phosphomevalonate kinase. Members of the GHMP superfamily have great three-dimensional similarity despite only ten to 20% sequence identity. These enzymes contain three well-conserved motifs (I, II, and III), the second of which is involved in nucleotide binding and has the sequence Pro-X-X-X-Gly-Leu-X-Ser-Ser-Ala.
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