Galactose is converted into glucose 1-phosphate (G1P) through a series of steps called the Leloir pathway. The first step of the pathway is the phosphorylation of galactose by galactokinase (encoded GALK1) to yield galactose 1-phosphate. Conversion of galactose 1-phosphate to G1P requires the transfer of UDP from UDP-glucose catalyzed by GALT. UDP-galactose is converted to UDP-glucose by GALE. Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase (PGM) and vice versa.
There are two known disorders concerning the uptake transports of galactose (SGLT1 and GLUT2 deficiency) and three known disorders of galactose metabolism: galactokinase deficiency (GALK-D), galactose 1-phosphate uridyltransferase deficiency (galactosemia, GALT-D) and uridine diphosphate galactose 4-epimerase deficiency (GALE-D).
Among these, galactosemia is the most common and most severe. This pathway was inspired by Chapter 18, figure 18.3 of the book of Blau (4th edition; ISBN: 978-3-642-40337-8).