Enzyme Classification: EC 1.1.1.47
Nomenclature: This enzyme is also known by several other names, including D-glucose dehydrogenase, NAD$^+$-dependent glucose dehydrogenase, and glucose:NAD$^+$ 1-oxidoreductase. It is distinct from glucose 1-dehydrogenase (NADP$^+$), which utilizes NADP$^+$ as a cofactor (EC 1.1.1.118).
Function: Glucose 1-dehydrogenase (NAD$^+$) is an oxidoreductase enzyme that catalyzes the stereospecific oxidation of the anomeric (C1) carbon of D-glucose. It uses nicotinamide adenine dinucleotide (NAD$^+$) as an electron acceptor, reducing it to NADH.
Reaction: The enzyme facilitates the following reversible biochemical reaction: D-glucose + NAD$^+$ $\rightleftharpoons$ D-glucono-1,5-lactone + NADH + H$^+$ The product D-glucono-1,5-lactone is an intramolecular ester that readily and spontaneously hydrolyzes in aqueous solutions to D-gluconate (gluconic acid). Therefore, the overall net reaction is often depicted as the conversion of D-glucose to D-gluconate.
Substrates: The primary substrates for this enzyme are D-glucose and NAD$^+$.
Products: The direct products of the enzymatic reaction are D-glucono-1,5-lactone, NADH, and a proton (H$^+$). As D-glucono-1,5-lactone rapidly hydrolyzes, D-gluconate is considered the ultimate product of glucose oxidation by this enzyme.
Mechanism: As a member of the oxidoreductase class, Glucose 1-dehydrogenase (NAD$^+$) removes a hydride ion directly from the C1 hydroxyl group of D-glucose. This hydride is then transferred to the NAD$^+$ molecule, reducing it to NADH. This process forms an ester linkage at the C1 position, resulting in the cyclic D-glucono-1,5-lactone.
Biological Role and Occurrence: This enzyme is widely distributed in various organisms, particularly in prokaryotes, including numerous species of bacteria (e.g., Bacillus subtilis, Pseudomonas fluorescens, Gluconobacter oxidans). It is also found in some fungi and a limited number of plant species. In these organisms, its principal role often involves the initial step in the oxidative catabolism of glucose, providing D-gluconate as an intermediate for further metabolic pathways, such as the Entner-Doudoroff pathway or the pentose phosphate pathway. This process also generates reducing equivalents (NADH), which can be utilized for energy generation (e.g., in the electron transport chain) or in various biosynthetic reactions. In some bacteria, it plays a role in the utilization of glucose as a carbon and energy source, or in specific detoxification pathways.
Industrial and Analytical Applications: Due to its high specificity for D-glucose and its NAD$^+$-dependence, Glucose 1-dehydrogenase (NAD$^+$) is extensively employed in various biochemical and biomedical applications. It is a key component in diagnostic kits and biosensors designed for the quantitative measurement of glucose in biological samples, such as blood, urine, and cerebrospinal fluid. Its use offers an advantage over glucose oxidase-based systems in certain contexts because it does not consume molecular oxygen, which can be beneficial in anaerobic or oxygen-limited environments, or when oxygen interference with other assay components is a concern.