Multiple Energy Conservation Bypasses in Oxidative Phosphorylation of Plant Mitochondria

The electron transport chain (ETC) in the inner mitochondrial membrane of most eukaryotes consists of four large protein complexes, the rotenone-sensitive NADH dehydrogenase (complex I), succinate dehydrogenase (complex II), cytochrome bc1 complex (complex III) and cytochrome c oxidase (complex IV). The electron transport activity of complexes I, III, and IV (the latter two often jointly called the cytochrome pathway) is coupled to extrusion of protons across the inner mitochondrial membrane. The electrochemical proton gradient that is formed and maintained in the process is used by the ATP synthase (also called complex V) to make ATP (Web Figure 12.3.A). The plant ETC, however, is highly branched. As a consequence, the plant oxidative phosphorylation system has several alternative pathways of electron transport via type II NAD(P)H dehydrogenases and the alternative oxidase (AOX), and additionally, an alternative proton transporter called the uncoupling protein (UCP). These proteins mediate bypasses around the proton-translocating multiprotein complexes (I–V): type II NAD(P)H dehydrogenases bypass complex I, AOX bypasses complexes III and IV, and UCP bypasses the ATP synthase (Siedow and Umbach 1995; Vanlerberghe and McIntosh 1997; Møller 2001; Millenaar and Lambers 2003; Fernie et al. 2004; Rasmusson et al. 2004, 2008; Vercesi et al. 2006; Vanlerberghe 2013). The type II NAD(P)H dehydrogenases and the AOX transport electrons without pumping protons, whereas the UCP allows proton flow from the intermembrane space to the matrix without ATP synthesis. The consequence of this is that flux through the energy conservation bypasses will mediate respiration that does not contribute to ATP production. At the same time, type II NAD(P)H dehydrogenases and AOX will not be controlled by cellular adenylate status. This Web Topic will describe the special features of these energy bypass proteins and discuss the consequences of their existence to the plant.