Galactose dehydrogenase

Related Reading

Galactose (CH₂OH(CHOH)₄CHO) is a molecular formula of monosaccharides, C₆H₁₂O₆, which can be found in dairy products or sugar beets. Galactose is a monosaccharide composed of six carbons and one aldehyde. It is classified as aldose and hexose and is an important component of certain glycoproteins. Galactose is a component of lactose in mammalian milk. Polysaccharides composed of D-galactose have been found in snails, frog eggs and cow lungs. It is often present in the brain and nerve tissues in the form of D-galactoside.

Introductions about galactose

Galactose is often present in a variety of plant gums in the form of polysaccharides in the plant kingdom. For example, K-carrageenan in red algae is a polysaccharide composed of D-galactose and 3,6-lactose-D-galactose. Free galactose is present in the berries of the ivy. Galactose is white crystals. Both D-galactose and L-galactose exist naturally. D-galactose generally exists in milk as a structural part of lactose. The lactose in milk is broken down by the human body into glucose and galactose for absorption and utilization. Because it contains calories, it can also be used as a nutritional sweetener. Galactose is part of the lactose molecule and the other half is glucose. Under the catalysis of β-lactase, galactose can be obtained from the hydrolysis of lactose. Bifidobacterium fermented lactose to produce galactose, which is a component of cerebrosides in the cranial nervous system, and is closely related to the rapid growth of the baby's brain after birth.

Galactose dehydrogenase

In enzymology, galactose 1 dehydrogenase is an enzyme that catalyzes chemical reactions
D-galactose + NAD⁺ →D-galactono-1,4-lactone + NADH + H⁺
Therefore, the two substrates of the enzyme are D-galactose and NAD⁺, and its three products are D-galactose-1,4-lactone, NADH and H+. This enzyme belongs to the family of oxidoreductases, especially those that use NAD + or NADP⁺ as the acceptor to act on the CH-OH group of the donor. The systematic name of this enzyme class is D-galactose: NAD⁺ 1-oxidoreductase. Other commonly used names include D-galactose dehydrogenase, β-galactose dehydrogenase, and NAD⁺-dependent D-galactose dehydrogenase. This enzyme is involved in galactose metabolism. Galactose dehydrogenase (NADP⁺) can also react with D-arabinose, 6-deoxy-D-galactose and 2-deoxy-D-galactose.

Introductions about galactose dehydrogenase

Galactose dehydrogenase (GaDH) catalyzes the dehydrogenation of b-D-galactose pyranose to D-galactose-1,5-lactone and NADH in the presence of NAD⁺, acting on the C1 position of the sugar substrate. This enzyme is also usually not absolutely specific to NAD⁺, because although to a lesser extent, NAD P⁺ is also used. D-galactose is a better substrate. Although other sugars (such as L-arabinose, 2-deoxy-D-galactose) are less reactive, the enzyme of D-galactose is also not specific. The kinetic mechanism is ordered Bi-Bi, where NAD⁺ first binds to the enzyme. GaDH of Pseudomonas fluorescens is the best researched example, because it has been cloned and expressed in E. coli, and its complete nucleotide sequence has been determined. The active macromolecule has two binding sites and consists of two identical subunits, each of which is 33 kDa (304 amino acid residues). GaDH from Pseudomonas saccharophila has been studied to a lesser extent, but has been identified in plants (such as green peas, oranges, and arabidopsis), algae (such as Iris), and several mammals including human enzyme.

Applications

GaDH is an important analytical tool, because at alkaline pH, the product of galactose lactone will be hydrolyzed, so the reaction becomes irreversible. Therefore, the enzyme can be used to determine b-D-galactose and a-D-galactose, and then the latter can be converted into the former by an exogenous mutagenic enzyme. GaDH is also used to determine lactose. Lactose is hydrolyzed by lactase, and then coupled with GaDH acting on the produced b-D-galactose.