Take a look on reaction catalyzed by dihydrolipoyl dehydrogenase. It acts upon $NAD^+$ according to following equation: $NAD^+ + FADH_2\rightarrow NADH+H^++FAD$
Same way you can write other reactions, e.g. by pyruvate dehydrogenase: $Pyruvate+TDP\rightarrow CO_2+TDP-Hydroxyethyl$