Energy for Translocation

Active Transport. As above, the process depends on a carrier, but differs in that the carrier provides energy for translocation from regions of lower concentration to regions of higher concentration. 

An exported protein is thus pushed through the membrane by a SecA protein located on the cytoplasmic surface, rather than being pulled through the membrane by a protein on the periplasmic surface. This difference may simply reflect the need for the translocating ATPase to be where the ATP is. The transmembrane electrochemical potential can also provide energy for translocation of the protein, by an as yet unknown mechanism. 

ABC transporter. ABC transporters for ATP-binding cassette transporters are an ancient family of proteins found in most organisms from bacteria to mammals. They contain a transmembrane segment and an intracellular ATP binding motive, and are involved in the transmembrane transport of various metabolites. The energy for translocation comes from ATP hydrolysis. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

The intersubunit rotation is required for translocation as ribosomes trapped in the nonrotated state by an engineered intersubunit disulfide bridge fail in tRNA-mRNA movement. Real-time observation of intersubunit movement by fluorescence resonance energy transfer (FRET) showed that intersubunit movement occurs concomitantly with hybrid state formation, and that the rotated state can be trapped by the antibiotic viomycin. Similarly to the fluctuation of tRNAs between classical and hybrid states, single-molecule studies have detected spontaneous intersubunit movement where the 3 OS subunit fluctuates between a rotated... 

This family of ATP-dependent motors [EC 3.6.1.33] that forms ADP and orthophosphate during its energy-dependent translocation along the surface of microtubules. Unique cargo sites on dynein molecules allow for the specific transport of cellular organelles and other macro-molecular components. 

The term ion pump, synonymous with active ion-transport system, is used to refer to a protein that translocates ions across a membrane, uphill against an electrochemical potential gradient. The primary pumps do so by utilization of energy derived from various types of chemical reactions such as ATP hydrolysis, electron transfers (redox processes), and decarboxylations, or from the absorption of light (Table 1). Secondary pumps are symport and antiport systems that derive the energy for uphill movement of one species from a coupled downhill movement of another species. The electrochemical gradient driving the latter movement is often created by a primary pump. 

La liquid-ordered phase bilayers prepared from binary mixtures of the same phosphatidylcholine and cholesterol but is much slower ( 10 s ) in Lx liquid-ordered phase membranes prepared from sphingomyelin and cholesterol (55). The activation free energy for the process, which corresponds to the energy necessary to put the translocating lipid molecule at the bilayer mid-plane, is 100kJmol In contrast, the rate constant for transmembrane translocation of cholesterol may be 1 s (56). 

In contrast to active transport, passive transport as a whole does not involve energy consumption and, therefore, only can work down a concentration gradient (or other types of gradients, such as electrochemical potential, thermal, or pressure gradients). In other words, passive transport of molecules equalizes their chemical potential on both sides of the membrane. The process of passive transport can be subdivided into two different mechanisms passive diffusion and facilitated transport. Passive diffusion is a physico-chemical process, whereas in facilitated transport, molecules pass through the membrane via special channels or are translocated via carrier proteins. Both passive diffusion and facilitated transport, in contrast to active transport, follow a gradient, where facilitation merely lowers the activation energy for the transport process. 

The translocation step is probably the point at which prokaryotic and eukaryotic secretion differ most. The energy for this process may derive from different sources from the energy of protein synthesis in eukaryotes, and from protein synthesis and/or ATP hydrolysis and/or the membrane potential in prokaryotes. [In fact there is evidence for more than one secretion pathway in E. coli. The degree of coupling between translation and translocation may also be different in prokaryotes and eukaryotes (Section V,C).]... 

Instead relies on the vectorial transport of proteins which leads to the establishment of an electrochemical proton gradient across the cell membrane. The energy for proton translocation Is provided by light which Is absorbed by the chromophore of bacteriorhodopsin. 

---