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Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is an enzyme involved in breaking down glucose to obtain energy.
More specifically, in eukaryotes, this enzyme catalyzes the sixth step in glycolysis, converting glyceraldehyde 3-phosphate to D-glycerate 1,3-bisphosphate (1,3-BPG).
After a few steps in glycolysis where no energy has been produced - in fact, two molecules of ATP are consumed in the initial steps - GAPDH is involved in the first reaction harnessing some of the energy present in glucose.
The reaction occurs in two coupled steps: first, aldehyde is converted to a carboxylic acid in the presence of NAD+, and secondly a molecule of organic phosphate is added to this to generate the acyl-phosphate end-product.
The reaction starts between the aldehyde and one of the sulfhydryl groups in GAPDH to form a hemithioacetal. This is followed by its deprotonation, to facilitate the transfer of the hydride ion (H-) tightly bound to GADPH to NAD+, generating NADH and a thioester intermediate. The second step involves phosphorylation of the thioester to form 1,3-BPG and a cysteine residue, but only after NADH has been replaced by a second NAD+. This positive charge helps polarize the thioester, facilitating the reaction with the phosphate group.
The first step is thermodynamically favorable, whereas the second reaction is unfavorable. As a consequence, if these reactions occurred separately, the second stage would be extremely limiting due to a high activation energy. Instead, the key is to couple the two reactions with GAPDH, so that the energy generated by the favorable aldehyde oxidation can be used to trigger the formation of 1,3-BPG1,2 .
Once its function was believed to have been unveiled to the last detail, GAPDH was thrown in the “housekeeping gene” list and considered devoid of any significant research impact. However, recent studies have found that GAPDH is capable of multiple functions completely unrelated to its participation in glycolysis 3. These include membrane fusion and cytoskeleton dynamics, as well as DNA repair and RNA export. Researchers believe these functions are likely to be regulated by post-translational changes and cellular localization4.
GAPDH is found to be a sensor of intracellular and extracellular cell stress capable of activating pathways to recover from the insult or activate the cell death signaling pathways 3,4. In the cytoplasm, under normal physiological conditions, active GAPDH promotes a move from anaerobic respiration to the pentose phosphate pathway. However, under conditions of oxidative stress, this enzyme can be reversibly inactivated by S-thiolation of its sulfhydryl groups, allowing cells to switch between metabolic functions and ensure maintenance of a balanced oxidation/reduction state5. In contrast, increasing levels of GAPDH accumulating in the mitochondria can activate apoptotic mechanisms, resulting in permeabilization of the mitochondrial membrane after release of apoptosis-inducing factor and cytochrome c6.
These new multiple GAPDH functions, especially as they involve mechanisms leading to cell death4, may indicate that when expressed in abnormal levels this enzyme is involved in certain diseases3. There is some evidence to support this, particularly in neurodegenerative conditions. For example, increased levels of GAPDH are found in post-mortem brains of patients suffering from Parkinson’s7 and Alzheimer’s disease8. It has been tentatively suggested that GAPDH can interact with mutant huntingtin and β-amyloid peptides, respectively9,10.