Structure–Function Relationship in UCP1

Abstract

The function of uncoupling protein (UCP1) as a H transporter regulated by nucleotide binding is elucidated. H transport requires fatty acids (FA) with relatively wide structural tolerance. The nucleotide binding site is speci®c for purine nucleotides and tolerates a number of derivatives. The strong pH dependency facilitates regulation of nucleotide binding and thus H translocation. The structure-function relationship of UCP1 has been analysed by various probes and by mutagenesis. According to our model, FA are a cofactor in H transport, providing H shuttling carboxyl groups in the translocation channel. By mutagenesis, additional H translocating groups at both sides of the translocation channel were found. Two pH sensors, controlling nucleotide binding, were identi®ed in accordance with earlier postulates deduced from the pH dependence of nucleoside diphosphate (NDP) and nucleoside triphosphate (NTP). A common pH sensor E190 and a speci®c pH sensor H214 for triphosphates only, control access to the phosphate binding moiety. The three mitochondrial carrier family characteristic intrahelical arginines are essential for nucleotide binding. Mutagenesis of other charged residues reveals their role in structure stabilisation and/or has more generalised effects due to charge relay networks in UCP1.

Lack of activation of UCP1 in isolated brown adipose tissue mitochondria by glucose-O-ω-modified saturated fatty acids of various chain lengths

“…These data are consistent with our previous data for UCP1 in isolated mitochondria [16]. The data are not consistent with the "cofactor activation" model of UCP 1 function [17][18][19] but are comparable with the model that UCP 1 functions by flipping longchain fatty acid anions [13][14][15]. …”

“…The key piece of evidence for the cofactor/activation model comes from tabulated data cited in two reviews [17,18], stating that the "unflippable" glycolipid, glucose-O-ω-palmitate "activates" native UCP1 reconstituted into liposome membranes. We subsequently re-investigated this latter crucial piece of evidence for the cofactor/activation model by first synthesizing glucose-O-ω-palmitate [20] and then investigating the ability of the modified long-chain fatty acid to activate UCP1 [16].…”

“…The second model proposes that UCP1 acts as a proton conduit across the mitochondrial inner membrane and, importantly, that fatty acids act as cofactors/activators providing additional carboxyl moieties at key intra-membrane sites, thus enhancing the rate of proton conductivity through the membrane [17][18][19]. The key piece of evidence for the cofactor/activation model comes from tabulated data cited in two reviews [17,18], stating that the "unflippable" glycolipid, glucose-O-ω-palmitate "activates" native UCP1 reconstituted into liposome membranes.…”

The association between the val/ala-55 polymorphism of the uncoupling protein 2 gene and exercise efficiency

“…11 The transmembrane proton transport by uncoupling proteins seems to be dependent on the presence of free fatty acids (FFA) but their actual role is still unclear. 25 However, if fatty acids take an active part in the proton translocation, the fatty acid distribution between the intermembrane space and matrix and, in turn, fat oxidation may be affected by the level of UCP activity. In favour of a regulatory role of UCP2 and UCP3 in fat utilization are the observations that gene expression of UCP2 and UCP3 in the skeletal muscles is upregulated by energy restriction 26,27 and that UCP3 gene expression both in humans 28 and rodents 29 has been shown to increase by the elevation of circulatory FFA, per se, and by aerobic training.…”

The -Adrenergic Receptors and the Control of Adipose Tissue Metabolism and Thermogenesis

“…Today, although there is still much debate conceming'the specific mechanism by which UCPl promotes proton movement (Garlid This simple cartoon compares the relationship between mitochondrial respiration that is "coupled" to the production of ATP via the ATP synthase, vs. the UCP-dependent transfer of protons across the inner membrane down the electrochemical gradient, whose action yields heat as a result of a need for increased tbel consumption to maintain ATP levels. RECEPTORS & ADIPOSE TISSUE 311 et al, 1998;Bouillaud, 1999;Klingenberg et al, 1999), there is little dispute that this specialized member of the mitochondrial carrier family is responsible for the utilization of metabolic fuel to render mitochondrial metabolism inefficient, the byproduct of which is heat.…”