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Glyceraldehyde-3-phosphate Dehydrogenase

spinqualitylabelsall models Glyceraldehyde-3-phosphate Dehydrogenase Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Role in Glycolysis:

• The Steps:

In two coupled steps,

GAPDH catalyzes the conversion of glyceraldyhyde-3-phosphate at carbon 1 to 1,3-bisphosphoglycerate (1,3-BPG).

• The Reactions:

The reaction involving the GAPDH enzyme combines phosphorylation with oxidation in an overall endergonic reaction. (ΔG°'=+6.3 kJ/mol (+1.5 kcal/mol)) First, the oxidation of glyceraldyhyde-3-phosphate to D-glycerate 1,3-bisphosphate takes place, in which an aldehyde is converted to carboxylic acid ((ΔG°'=-50 kJ/mol (-12 kcal/mol))and NAD+ (Nicotinamide adenine dinucleotide), an important co-factor and ligand found bound to the of GAPDH, is simultaneously reduced endergonically to NADH. This oxidation reaction is required for the initiation of the second reaction because it is highly exergonic and thus drives the endergonic second reaction ((ΔG°'=+50 kJ/mol (+12 kcal/mol)). In the second reaction a molecule of inorganic phosphate is transferred to a GAP intermediate to form a product with a high potential to transfer phosphates, 1,3-bisphosphoglycerate.

• The Mechanism:

The mechanism of GAPDH is mainly dependent the thiol functional group on a cysteine residue which lies within the . This important thiol group acts as a nucleophile, attacking the carbon on the aldehyde functional group of the substrate, glyceraldehyde 3-phosphate, after it binds to GAPDH. The creation of a thiohemiacetal intermediate occurs from this oxidative reaction, which then loses a hydride to NAD+, which is also bound nearby, forming NADH and a carboxyl group from the aldehyde. Conservation of energy is maintained through this thioester linkage to the active site cysteine. The GAP intermediate is then used in the second step, where it is important to notice that without GAPDH's use of covalent catalysis, the energy barrier of the reaction would be too high and the reaction would be too slow for living organisms.

Other roles:

• The importance of GAPDH in transcription was discovered by Zheng et. al. in 2003. It was found that OCA's transcriptional coactivator complex contains GAPDH, which moves between the cytosol and nucleus and may link the metabolic state to gene transcription.
• In 2005 the initiation of apoptosis was shown to be mediated by GAPDH by Hara et. al. when it was found to bind to DNA like it does in transcription activation.

GAPDH was also found to be involved in ER to Golgi transport because it is recruited by rab2 to vesicular-tubular clusters of the endoplasmic reticulum where it helps form COP 1 vesicles.

• It was also found that GAPDH is plays a role in certain neruodegenerative disorders, as is able to find stretches which are encoded by the gene's CAG repeats and bind to the gene products formed from disorders such as Huntington's disease, Alzheimer’s disease, Parkinson’s disease and Machado-Joseph disease.

Use in the lab:

• Overall, GAPDH is a “housekeeping gene” and is found in high levels in tissues and cells so it is commonly used in biological research as a loading control in western blot and RT-PCR, but it has to be carefully controlled because under specific conditions it can have various regulation.

References

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