Fig. 1.

Uncoupling protein 1 (UCP1)-dependent and independent thermogenesis in brown and beige adipocytes. UCP1, a protein located in the mitochondrial inner membrane, has the ability to leak proton gradient across the mitochondrial membrane, and thereby uncouples oxidative phosphorylation to generate heat. Thus, UCP1 thermogenesis is not dependent on adenosine triphosphate (ATP). On the other hand, thermogenesis by the futile cycles of creatine and creatine phosphate, release and re-uptake of calcium, and triglyceride (TG) hydrolysis and fatty acid (FA) re-esterification is dependent on the breakdown of ATP to adenosine diphosphate (ADP). These thermogenic mechanisms are activated by cold exposure primarily through the sympathetic nerve activation. RyR, ryanodine receptor; SERCA, sarco-endoplasmic reticulum ATPase.