Is a coupled rxn any rxn that combines 1 endergonic and 1
exergonic rxns? So then in glycolysis, step 6, GA3P ---> 1,3diPGA plus NAD--->NADH2;
and before the Krebs cycle, pyr ---> acetyl-CoA plus NAD reduction; and in the Krebs
cycle, isocitric acid ---> keto plus NAD reduction... Are all these three rxns examples
of coupling?
In the broadest sense, we can talk about two reactions coupled to contrast the same two
reactions not coupled, that is, occuring independently. This is easy to see for the first
such coupled reaction we considered, the phosphorylation of glucose by ATP instead by
phosphate addition. For the cases you cite, it is a little more difficult to see the
reactions independently, since they are oxidation-reductions and so are obligatorily
coupled. Nevertheless, in English terms, we can talk abstractly about the reduction of NAD
to NADH2, and the oxidation of pyruvate to acetyl-CoA and CO2, as separate
"paper" reactions, that are then coupled in the actual reaction leading to the
Krebs cycle.