Active transport group translocation

Group Iruudoeatiom is a special type of membrane T. A membrane-bound enzyme catalyses a reaction between substrates on opposite sides of the membrane, and the product accumulates on one side of the membrane, e.g. various bacteria take up glucose by phosphoiylating it to glucose 6-phosphate. Group translocation is also involved in the T. of amino acids. In contrast to active T., group translocation involves modification of the transported material. 

Yarbrough, J. M., Rake, J. B., and Eagon, R. G. (1980). Bacterial inhibitory effects of nitrite Inhibition of active transport, but not of group translocation, and of intracellular enzymes. Appl. Environ. Microbiol. 39, 831-834. 

One kind of active transport, namely group translocation, occurs in bacteria (for reviews, see Refs. 218-223), and some workers consider that this also takes place in yeasts (see, for example, Refs. 224 and 225) by this means, uptake of a sugar is directly coupled to its phosphorylation, and the sugar is released into the cytoplasm as a phosphate. 

Alternatively, both the first and the second solutes may pass through the membrane bound to the same carrier (cotransport or symport). Another form of active transport is group translocation, a process in which the substance to be transported undergoes covalent modification, e.g., by phosphorylation. Tire modified product enters the cell and within the cell may be converted back to the unmodified substance. Transport processes, whether facilitated or active, often require the participation of more than one membrane protein. Sometimes the name permease is used to describe the protein complexes utilized. 

In the bacterium E. coli, glucose is taken up by group translocation, lactose is taken up by secondary active transport (using H + ), and maltose is taken up by means of a binding-protein system. Outline how it would be possible to determine whether melibiose (a disaccharide of glucose and galactose) is taken up by . coli and. if it is, whether one of the mechanisms described earlier applies. 

Group translocation An active transport process in bacteria that chemically modifies substance so it cannot diffuse out of the cell. 

In the actual state of our knowledge we should carefully avoid the application of the definition of active transport, given below, to any translocation of a chemical group or radical from a donor to an acceptor. More appropriately, we should consider some of the relationships established between an organism and its surroundings For instance, how does a frog, sitting in tap-water, keep its interior of different ionic composition (Na and a ) from the outside medium ... 

The topic of membrane transport is discussed in detail in many texts (Alberts et al, 1989 Freshney, 2000). The following discussion is limited to membrane transport of small molecules, hence excluding macromolecules such as polypeptides, polysaccharides, and polynucleotides. The lipid bilayer is a highly impermeable barrier to most polar molecules and thus prevents the loss of the water-soluble contents of the cell interior. Consequently, cells have developed special means to transport these species across their membranes. Specialized transmembrane proteins accomplish this, each responsible for the transfer of a specific molecule or group of closely related molecules. The mechanism can be either energy independent, as in passive and facilitated diffusion, or energy dependent, as in active transport and group translocation. 

Taken together, these results indicate that similar to other proton-translocating membrane proteins, both types of Na /H exchangers contain critical sulfhydryl groups that are involved in the transport mechanism. These sulfhydryl groups do not appear to be present at the external transport site but may be involved in switching from an inactive to an activated state. 

RIPs are plant protein toxins that are able to inhibit enzymatically ribosomal activity and are therefore highly cytotoxic [98]. RIPs are taken up in the cells by means of endocytosis, and only a small fraction (5% or less) are translocated to the cytosol where the toxins inhibit the protein synthesis and eventually kill the cell. PCI may be used to increase both the efficacy and specificity of these toxins. RIPs are divided into two groups, type I and type II. Type II RIPs, like ricin, consists of two polypeptide chains, one cytotoxic A-chain with /V-glycosidase activity and one B-chain which binds to the cell surface. Type I RIPs, like gelonin, agrostin, and saporin, lack the B chain, which make them poorly transported over the cell- and intracellular membranes to the cell cytosol. Hence, the cytotoxic effect of these protein toxins is usually absent or very low. A considerable cytotoxic effect of type I RIPs has been shown in combination with PCI, both in vitro and in vivo [25, 99]. 

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