NAD+ is reduced to NADH in the reactions below:
NAD+ + pyruvate + CoA -> NADH + H+ + CO2 + Acetyl-CoA
3rd step of Kreb's cycle
NAD+ + Isocitrate -> NADH + H+ + CO2 + α-ketoglutarate
4th step of Kreb's cycle
NAD+ + α-ketoglutarate + CoA -> NADH + H+ + CO2 + Succinyl-CoA
Is there a reason why CO2 is produced when there is a reduction of NAD+?
The texts I have so far simply state that CO2 is derived from the carbon atom that is lost in the hydrocarbon molecule. However, that doesn't seem to explain the core reason why this happens.