biochemistry - What exothermic reaction distinguishes warm blooded animals?

While all animals and even plants generate heat from chemical reactions or mechanical motion of their tissues (like muscle tissues), warm blooded animals have brown fat which has the unusual ability to generate heat directly from metabolic energy.

The particular chemical process you are looking for is performed by uncoupling proteins (UCPs). They reside in the mitochondria and rather than use the proton gradient in the mitochondria to generate ATP, UCPs pass protons through the membrane and generate heat.

Brown fat is brown because of a relatively large iron content associated with many extra mitochondria (brown adipose tissue appears to be derived from muscle cells and not white adipose tissue). Its a heat generating organ in humans, particularly infants.