Fluorescent nucleotide analogs as active site probes for the ADP/ATP carrier and the uncoupling protein

Uncoupling proteins: Martin Klingenberg's contributions for 40 years

The uncoupling protein (UCP1) is a proton (H⁺) transporter in the mitochondrial inner membrane. By dissipating the electrochemical H⁺ gradient, UCP1 uncouples respiration from ATP synthesis, which drives an increase in substrate oxidation via the TCA cycle flux that generates more heat. The mitochondrial uncoupling-mediated non-shivering thermogenesis in brown adipose tissue is vital primarily to mammals, such as rodents and new-born humans, but more recently additional functions in adult humans have been described. UCP1 is regulated by β-adrenergic receptors through the sympathetic nervous system and at the molecular activity level by nucleotides and fatty acid to meet thermogenesis needs. The discovery of novel UCP homologs has greatly contributed to the understanding of human diseases, such as obesity and diabetes. In this article, we review the progress made towards the molecular mechanism and function of the UCPs, in particular focusing on the influential contributions from Martin Klingenberg's laboratory. Because all members of the UCP family are potentially promising drug targets, we also present and discuss possible approaches and methods for UCP-related drug discovery.

EPR spectroscopy of 5-DOXYL-stearic acid bound to the mitochondrial uncoupling protein reveals its competitive displacement by alkylsulfonates in the channel and allosteric displacement by ATP

Competition of fatty acids (FA) and alkylsulfonates with 5-DOXYL-stearic acid (5-SASL) binding to isolated mitochondrial uncoupling protein (UcP) is demonstrated using EPR spectroscopy. A distinct peak of the bound 5-SASL (h+1I) decreased with increasing concentration of competitors. Since alkylsulfonates are UcP substrates, it suggests that the FA binding site is located in the anion channel. Moreover, with increasing ATP the h+1I peak decreased and was smoothed with the 'micellar' peak into a single wider peak. A pH of 8.5 reversed this effect. It could reflect an allosteric release of 5-SASL from the ATP binding site which mimics the ATP gating mechanism.

Binding of ATP to uncoupling protein of brown fat mitochondria as studied by means of spin-labeled ATP derivatives

ATP derivatives spin-labeled (SL) at C8, N6, C2' or C3' were employed in binding studies with the uncoupling protein of brown fat mitochondria. Substitution of the ribose strongly impaired binding, whereas labeling of the adenine moiety allowed for tight and functional complex formation. Detailed binding studies with C8-SL-ATP confirmed the known pH and Mg2+ dependence with a stoichiometry of one C8-SL-ATP bound per 66 kDa dimer. Corresponding studies of the uncoupling protein after modification with N-ethylmaleimide or diazobenzene-4-sulfonic acid revealed distinct differences in their effects on nucleotide binding and gating.

Distinct affinity and effector residues in the binding site for a regulatory ligand. The mitochondrial uncoupling protein as a model

A hypothesis concerning two distinct classes of amino acid residues in some regulatory binding sites is proposed. The "affinity residues" are those that are unable to transduce the ligand information signal but are responsible for overcoming the barrier for the attachment of a ligand to its binding site while the "effector residues" transfer the binding signal to the other functional part of the protein, which then undergoes a non-equilibrium energetic cycle induced by interaction with the ligand. As an example, the purine nucleotide inhibition of H+ transport through the uncoupling protein of brown adipose tissue mitochondria is discussed; there is a concentration range in which the nucleotide is bound but does not inhibit H+ transport. This is interpreted in terms of inaccessibility of the effector residues inducing H+ transport inhibition below a certain threshold concentration.