Functional binding of cardiolipin to cytochrome c oxidase

Forces and factors that contribute to the structural stability of membrane proteins

While a considerable amount of literature deals with the structural energetics of water-soluble proteins, relatively little is known about the forces that determine the stability of membrane proteins. Similarly, only a few membrane protein structures are known at atomic resolution, although new structures have recently been described. In this article, we review the current knowledge about the structural features of membrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-c oxidase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements in membrane proteins. This property manifests itself as incomplete unfolding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief general account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.

Inhibition of cardiolipin biosynthesis in the hypoxic rat heart

Cardiolipin is the principal polyglycerophospholipid in the heart. The effect of hypoxia on cardiolipin biosynthesis was investigated in isolated rat hearts perfused in the Langendorff mode. Hearts were pulsed-labeled for 60 min with 0.1 mM [1,(3)-3H]glycerol in Krebs Henseleit buffer saturated with either 95% O2/5% CO2 (control) or 95% N2/5% CO2 (hypoxic). Radioactivity incorporated into phosphatidylglycerol and cardiolipin were reduced 88% (P < .05) and 79% (P < .05), respectively, in hypoxic hearts compared to controls. In other experiments, hearts were pulse-labeled for 15 min with 1.4 mM [32P]Pi in Krebs Henseleit buffer saturated with 95% O2/5% CO2 and subsequently perfused for 60 min under control or hypoxic conditions. The radioactivity incorporated into CDP-1,2-diacyl-sn-glycerol, phosphatidylglycerol, and cardiolipin were reduced 61% (P < .05), 71% (P < .05), and 70% (P < .05), respectively, in the hypoxic hearts compared to controls, indicating a decreased formation of CDP-1,2-diacyl-sn-glycerol in the hypoxic heart. The activities of the enzymes involved in cardiolipin biosynthesis and the cardiac pool sizes of cardiolipin, phosphatidylglycerol, and CDP-1,2-diacyl-1,2-diacyl-sn-glycerol were unaltered between hypoxic and control hearts. In contrast, cardiac adenosine-5'-triphosphate and CPT levels were decreased 94% (P < .05) and 92% (P < .05), respectively, in hypoxic hearts compared to controls. We postulate that the biosynthesis of the cardiac polyglycerophospholipid cardiolipin may be inhibited by a decreased adenosine-5'-triphosphate and cytidine-5'-triphosphate level in the heart.

Forces and factors that contribute to the structural stability of membrane proteins

While a considerable amount of literature deals with the structural energetics of water-soluble proteins, relatively little is known about the forces that determine the stability of membrane proteins. Similarly, only a few membrane protein structures are known at atomic resolution, although new structures have recently been described. In this article, we review the current knowledge about the structural features of membrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-c oxidase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements in membrane proteins. This property manifests itself as incomplete unfolding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief general account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.