Step 5. Oxidative Phosphorylation of Glyceraldehyde-3-Phosphate

There are two aspects to this process (a) the oxidation and subsequent phosphorylation of glyceraldehyde-3-phosphate (G3P) to 1,3-bisphospho-glycerate (1,3-BGP), and (b) the reduction of nicotinamide adenine dinucleotide (NAD+) to its reduced form designated as NADH. The net reaction can be written as 

Inspection of the three-carbon reactant (G3P) and product species (1,3-BGP) given below shows that the phosphate group is present in the reactant, G3P, as an ester of an alcoholic oxygen, while in the product, 1,3-BGP, it is involved in an anhydride linkage with an acyl group. 

The oxidation and reduction steps are coupled through the binding of G3P to a sulfhydryl group on an enzyme that already has bound a NAD+ molecule. The enzyme facilitates a H- (hydride ion) shift from G3P to reduce the NAD+ to NADH. The oxidation of G3P results in the formation of a high-energy thioester bond that is subsequently replaced by an orthophosphate group to yield the 1,3-BGP. 

Because one glucose molecule yields two three-carbon fragments, each of which goes on to become pyruvate, there is a net synthesis of two ATP molecules for each glucose molecule consumed. 

Heat and work can be distinguished in terms of random molecular motion versus directed or coordinated motion. In muscle cells, from organisms as simple as yeast to those as complex as humans, the hydrolysis of ATP provides the driving force for the interactions and conformation of two cellular proteins, myosin and actin. Conformational changes associated with the binding and release of ATP and ADP provide the means by which a coordinated movement of these muscle cells is possible to do mechanical work. 

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