GPDH

Glycerol-3-phosphate dehydrogenase (GPDH) is an enzyme reversibly catalyzing the reduction of dihydroxyacetone phosphate (DHAP) from the glycolytic pathway to glycerol 3-phosphate that is important for lipid biosynthesis. GPDH has previously been termed as alpha glycerol-3-phosphate dehydrogenase (alphaGPDH) and glycerolphosphate dehydrogenase. GPDH is also referred to as DHAP reductase in algae and higher plants by reacting specifically with nicotinamide adenine dinucleotide (NADH) and DHAP as substrates. GPDH functions as a main link between carbohydrate metabolism and lipid metabolism and also makes prominent contributions to the electron transport chain in the mitochondria.

Structure

Glycerol-3-phosphate dehydrogenase contains two protein domains, where the N-terminal domain is responsible for NAD-binding, and the C-terminus serves as a substrate-binding fragment. However, dimer and tetramer interface residues are associated with glyceraldehyde 3-phosphate dehydrogenase (GAPDH)-RNA binding, since GAPDH demonstrates several moonlighting activities, including the modulation of RNA binding and/or stability.

Variants of GPDH

There are two forms of GPDH: GPD1 and GPD2.

a. GPD1

Cytosolic glycerol-3-phosphate dehydrogenase (GPD1) consists of two subunits and is a NAD⁺-dependent enzyme conducting reduction of dihydroxyacetone phosphate to glycerol-3-phosphate. At the same time, NADH is oxidized to NAD⁺ in the following reaction. Consequently, NAD⁺ is regenerated for further metabolic activity.

Figure 1. GPD1 reaction mechanism.

b. GPD2

Mitochondrial glycerol-3-phosphate dehydrogenase (GPD2) consists of 4 identical subunits and can catalyze the irreversible oxidation of glycerol-3-phosphate to dihydroxyacetone phosphate. Concomitantly, two electrons from flavin adenine dinucleotide (FAD) are transferred to the electron transport chain.

Studies have indicated that GPDH is mostly free from the influence of pH changes. The activity of GPD1 and GPD2 is poor under certain pH conditions. High salt concentrations lead to accumulation of glycerol, thus causing a higher increase in GPD1 activity than GPD2. Changes in temperature appear not to support GPD1 and GPD2.

Figure 2. GPD2 reaction mechanism.

Glycerol-3-phosphate Shuttle

The cytosolic GPDH and the mitochondrial GPDH function in concert. Oxidation of cytoplasmic NADH by the cytosolic enzyme produces glycerol-3-phosphate from dihydroxyacetone phosphate. Once moving through the inner mitochondrial membrane, glycerol-3-phosphate can then be oxidized by a separate isoform of GPDH that employs quinone as an oxidant and FAD as a cofactor. Accordingly, a net loss in energy occurs, comparable to one molecule of ATP. The combined action of these enzymes keeps the NAD⁺/NADH ratio stable, allowing for continuous operation of metabolism.

Reaction

The NAD⁺/NADH coenzyme couple is an electron reservoir to transport electrons from one metabolic redox reaction to another, most of which occur in the mitochondria. In order to regenerate NAD⁺ for further use, NADH present in the cytosol must be reoxidized. The mitochondrial inner membrane is impermeable for both NADH and NAD⁺, thereby they cannot be freely exchanged between the cytosol and mitochondrial matrix. Glycerol-3-phosphate is a critical tool to shuttle this reducing equivalent across the membrane by employing the two forms of GPDH. Cytosolic GPDH (GPD1) located in the mitochondrial inner space or cytosol could catalyze the reduction of dihydroxyacetone phosphate into glycerol-3-phosphate. Jointly, mitochondrial GPDH (GPD2) embedded on the outer surface of the inner mitochondrial membrane, far from the cytosol, accelerates the oxidation of glycerol-3-phosphate to dihydroxyacetone phosphate. The reactions catalyzed by cytosolic (soluble) and mitochondrial GPDH are shown below:

Figure 3. Coupled reactions catalyzed by GPD1 and GPD2. (Kota V; et al. 2010)

Metabolic function

GPDH is of great significance in lipid biosynthesis. GPDH allows a prompt dephosphorylation of glycerol 3-phosphate into glycerol through the reduction of dihydroxyacetone phosphate into glycerol 3-phosphate. Moreover, GPDH is one of the enzymes that are implicated in maintaining the redox potential across the inner mitochondrial membrane.

Role in Diseases

The fundamental roles of GDPH in keeping the NAD⁺/NADH potential and lipid metabolism, enable GDPH a contributor to lipid imbalance diseases, such as obesity. An enhancement in GPDH activity, particularly GPD2, could result in an increase in glycerol production. Glycerol abundance can readily initiate increase in triglyceride accumulation at a cellular level, which presents a tendency to form adipose tissue with an accumulation of fat that favors obesity. GPDH has also been involved in Brugada syndrome. Mutations in the gene encoding GPD1 could induce defects in the electron transport chain. This conflict with NAD⁺/NADH levels in cell is thought to cause deficiencies in cardiac sodium ion channel regulation and can result in a lethal arrythmia during infancy.

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