Obesity increases the risk of mortality because of metabolic sequelae such as type 2 diabetes and cardiovascular disease¹. Thermogenesis by adipocytes can counteract obesity and metabolic diseases^2,3. In thermogenic fat, creatine liberates a molar excess of mitochondrial ADP-purportedly via a phosphorylation cycle⁴-to drive thermogenic respiration. However, the proteins that control this futile creatine cycle are unknown. Here we show that creatine kinase B (CKB) is indispensable for thermogenesis resulting from the futile creatine cycle, during which it traffics to mitochondria using an internal mitochondrial targeting sequence. CKB is powerfully induced by thermogenic stimuli in both mouse and human adipocytes. Adipocyte-selective inactivation of Ckb in mice diminishes thermogenic capacity, increases predisposition to obesity, and disrupts glucose homeostasis. CKB is therefore a key effector of the futile creatine cycle.

In eukaryotes, cellular respiration is driven by mitochondrial cytochrome c oxidase (CcO), an enzyme complex that requires copper cofactors for its catalytic activity. Insertion of copper into its catalytically active subunits, including COX2, is a complex process that requires metallochaperones and redox proteins including SCO1, SCO2, and COA6, a recently discovered protein whose molecular function is unknown. To uncover the molecular mechanism by which COA6 and SCO proteins mediate copper delivery to COX2, we have solved the solution structure of COA6, which reveals a coiled-coil-helix-coiled-coil-helix domain typical of redox-active proteins found in the mitochondrial inter-membrane space. Accordingly, we demonstrate that COA6 can reduce the copper-coordinating disulfides of its client proteins, SCO1 and COX2, allowing for copper binding. Finally, our determination of the interaction surfaces and reduction potentials of COA6 and its client proteins provides a mechanism of how metallochaperone and disulfide reductase activities are coordinated to deliver copper to CcO.

Noradrenaline (NA) regulates cold-stimulated adipocyte thermogenesis¹. Aside from cAMP signalling downstream of β-adrenergic receptor activation, how NA promotes thermogenic output is still not fully understood. Here, we show that coordinated α₁-adrenergic receptor (AR) and β₃-AR signalling induces the expression of thermogenic genes of the futile creatine cycle^2,3, and that early B cell factors, oestrogen-related receptors and PGC1α are required for this response in vivo. NA triggers physical and functional coupling between the α₁-AR subtype (ADRA1A) and Gα_q to promote adipocyte thermogenesis in a manner that is dependent on the effector proteins of the futile creatine cycle, creatine kinase B and tissue-non-specific alkaline phosphatase. Combined Gα_q and Gα_s signalling selectively in adipocytes promotes a continual rise in whole-body energy expenditure, and creatine kinase B is required for this effect. Thus, the ADRA1A-Gα_q-futile creatine cycle axis is a key regulator of facultative and adaptive thermogenesis.