Galactokinase

Galactokinase 1
Cartoon structure of a human galactokinase 1 monomer in complex with galactose (red) and an ATP analogue (orange). A magnesium ion is visible as a green sphere. (From PDB: 1WUU​)
Identifiers
Symbol	GALK1
Alt. symbols	GALK
Entrez	2584
HUGO	4118
OMIM	604313
RefSeq	NM_000154
UniProt	P51570
Other data
EC number	2.7.1.6
Locus	Chr. 17 q23-q25

Galactokinase is an enzyme (phosphotransferase) that facilitates the phosphorylation of α-D-galactose to galactose 1-phosphate at the expense of one molecule of ATP. Galactokinase catalyzes the second step of the Leloir pathway, a metabolic pathway found in most organisms for the catabolism of β-D-galactose to glucose 1-phosphate. First isolated from mammalian liver, galactokinase has been studied extensively in yeast,archaea,plants, and humans.

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.