Galactan is a
polymeric form of galactose found in
hemicellulose, and forming the core of the galactans, a class of natural polymeric carbohydrates. D-Galactose is also known as brain sugar since it is a component of
glycoproteins (oligosaccharide-protein compounds) found in
nerve tissue. Galactofuranose occurs in bacteria, fungi and protozoa, and is recognized by a putative chordate immune lectin
intelectin through its exocyclic 1,2-diol.
Relationship to lactose Galactose, when combined with glucose (another monosaccharide) through a
condensation reaction, gives the disaccharide called
lactose. The
hydrolysis of lactose to glucose and galactose is
catalyzed by the
enzymes
lactase and
β-galactosidase. The latter is produced by the
lac operon in
Escherichia coli. In nature, lactose is found primarily in milk and milk products. Consequently, various food products made with dairy-derived ingredients can contain lactose. Galactose is
metabolised to glucose by the three principal enzymes in a mechanism known as the
Leloir pathway. The enzymes are listed in the order of the metabolic pathway:
galactokinase (GALK),
galactose-1-phosphate uridyltransferase (GALT), and
UDP-galactose 4′-epimerase (GALE). In human
lactation, galactose is required in a 1 to 1 ratio with glucose to enable the
mammary glands to synthesize and secrete lactose. In a study where women were fed a diet containing galactose, 69 ± 6% of glucose and 54 ± 4% of galactose in the lactose they produced were derived directly from
plasma glucose, while 7 ± 2% of the glucose and 12 ± 2% of the galactose in the lactose, were derived directly from plasma galactose. 25 ± 8% of the glucose and 35 ± 6% of the galactose was synthesized from smaller molecules in a process referred to in the paper as
hexoneogenesis. This suggests that the synthesis of galactose is supplemented by direct uptake and of use of plasma galactose when present. ==Structure and isomerism==