In general, bond forming is an exothermic process (releases energy/heat) and bond breaking is an endothermic process (absorbs energy/heat). As you are no doubt aware, the energy currency of the body is ATP. That is because the attachment of a phosphate to things releases a lot of energy, so any time you go from adenosine diphosphate to adenosine triphosphate/adenine, you are making a lot of heat.

The other major energy source coming out of the Krebs cycle is the redox reaction of nicotinamide derivatives (NADH, etc)

I believe ###### and ###### asked me something similar recently, and I told them it was phosphates attaching.