Atp as a Source of Energy for Periplasmic Permease in the Membrane Transport Systems
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The many components of cells are separated by membrane bound compartments such as periplasm, inner membrane and cytoplasm. Molecules may cross these membranes by simple or facilitated diffusion; however, active transport with ATP is required to drive the transport of large, highly charged and highly hydrophobic molecules against their concentration gradient. This energy requiring process couples ATP hydrolysis with transport proteins to bypass the impermeable nature of membranes (Wikipedia, Nov. 14, 2005, electronic communication). In this paper, we will explore and demonstrate how transport proteins achieve active transport through protein conformation changes and protein phosporylation .
Transport protein forms on transmembrane or membrane-crossing domains (TMD) that span the membrane successively as α-helices (see Fig 1) (Higgins, 2001). For each domain, there is a hydrophilic and peripherally associated ATP or nucleotide binding domain (NBD) (see Fig 2) (Higgins, 2001). The transport cycle initiates when a substrate binds to the TMDs and interacts at the extracellular face of the membrane. The binding of the substrate induces a conformational change at the TMDs. This change in turn retransmits to the NBDs, changing its conformation and indirectly inducing the initiating of ATP hydrolysis. So we can see that both NBDs and TMDs must be present for the hydrolysis of ATP (Higgins, 2001). The hydrolysis of ATP induces further conformational change at the NBDs, the effect is passed on to TMDs which reverses to its original shape(Higgins, 2001). These steps mark the cyclical process of active transport.
An experiment conducted by Bishop and his associates demonstrated that ATP is indeed the source of energy used by transport proteins for the active transport of molecules across the membrane. Their study uses a transport enzyme, namely histidine permease, which is comprised of the histidine-binding protein (transport protein), HisJ, two hydrophobic integral membrane proteins (TMDs), HisQ and HisM, and the nucleotide-binding membrane protein (NBDs), HisP (Bishop et al. 1989). They began selecting a model system that would separate each of the permease proteins mentioned above. Three systems were tested one after the other in attempt to purify the components. The first two in intro systems failed to extract several proteins embedded in membrane vesicles; a third technique, reconstituted proteoliposomes, is a newly developed method that reconstitutes the histidine permease into proteoliposomes (Bishop et al. 1989). The solution of proteins could then be analysized by Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis. Results show that proteoliposomes do not catalysis the hydrolysis of ATP to a large degree until histidine transport is first exposed to substrate along with HisJ (Bishop et al. 1989). These results