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NAD is commonly called by other names, including NAD+ or NADH. These are both forms of NAD — NAD+ is the positively charged form, which has lost an electron, and NADH is the neutral form which ...
The effects of the NAD + /NADH ratio are complex, controlling the activity of several key enzymes, including glyceraldehyde 3-phosphate dehydrogenase and pyruvate dehydrogenase. In healthy mammalian tissues, estimates of the ratio of free NAD + to NADH in the cytoplasm typically lie around 700:1; the ratio is thus favorable for oxidative reactions.
[10] While under standard conditions malate cannot reduce the more electronegative NAD +:NADH couple, in the cell the concentration of oxaloacetate is kept low enough that Malate dehydrogenase can reduce NAD + to NADH during the citric acid cycle. Fumarate + 2 H + + 2 e − → Succinate +0.03 [9] O 2 + 2H + + 2e − → H 2 O 2 +0.30
The change in free energy, ΔG, for each step in the glycolysis pathway can be calculated using ΔG = ΔG°′ + RTln Q, where Q is the reaction quotient. This requires knowing the concentrations of the metabolites. All of these values are available for erythrocytes, with the exception of the concentrations of NAD + and NADH.
The energy from the acetyl group, in the form of electrons, is used to reduce NAD+ and FAD to NADH and FADH 2, respectively. NADH and FADH 2 contain the stored energy harnessed from the initial glucose molecule and is used in the electron transport chain where the bulk of the ATP is produced. [1]
The glycerol-3-phosphate shuttle is a mechanism used in skeletal muscle and the brain [1] that regenerates NAD + from NADH, a by-product of glycolysis. NADH is a reducing equivalent that stores electrons generated in the cytoplasm during glycolysis. NADH must be transported into the mitochondria to enter the oxidative phosphorylation pathway.
Reverse electron transfer (RET) is the process that can occur in respiring mitochondria, when a small fraction of electrons from reduced ubiquinol is driven upstream by the membrane potential towards mitochondrial complex I. This results in reduction of oxidized pyridine nucleotide (NAD + or NADP +).
One mechanism, which is also the most naturally present pathway, involves electron transfer catalyzed by cytochrome b5 reductase through the oxidation of NADH to NAD+. The electron donor, NADH, that supports this reaction is a product of glucose oxidation from glycolysis .