Amino acids coupled transport

Figure 8.5 Act/Ve amino add transport into cells. Amino acids are transported into cells against their concentration gradient coupled to Na ion transport down its concentration gradient. The Na ion is transported out in exchange for K -ions, via the NayK ATPase, Chapter 5).

On the other hand, amino acids are transported inside the cell by different transporters. To date, 15 transport systems have been identified for amino acids in Sac-charomyces cerevisiae (Barre et al. 1998) and all of them are symport systems coupled to the entry of a proton. This proton must also be sent outside the cell in order to maintain the cellular homeostasis. Therefore, the uptake of ammonium and amino acids must be considered as active transport because it consumes ATP via H+-ATPase. 

Intracellular metabolism of amino acids requires their transport across the cell membrane. Transport of L-amino acids occurs against a concentration gradient and is an active process usually coupled to Na -dependent carrier systems as for transport of glucose across the intestinal mucosa (Chapter 12). At least five transport systems for amino acids (with overlapping specificities) have been identified in kidney and intestine. They transport neutral amino acids, acidic amino acids, basic amino acids, ornithine and cystine, and glycine and proline, respectively. Within a given carrier system, amino acids may compete for transport (e.g., phenylalanine with tryptophan). Na+-independent transport carriers for neutral and lipophilic amino acids have also been described, d-Amino acids are transported by simple diffusion favored by a concentration gradient. 

Protein biosynthesis occurs in ribosomes, the cellular assembly factory where amino acids are covalently linked into a polypeptide chain along the template mRNA. The amino acids are transported into the ribosome in an activated state via coupling to specific carriers known as transfer RNAs (tRNA). The selection of specific amino acids from the cellular pool of amino acids (Tables 1.2 and 1.3) and the task of charging to cognate tRNAs are performed by specific aminoacyl-... 

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

Despite the limited information available, rather clear predictions can be made about the probable structure, location, and energy coupling of the amino acid transporters of Saccharomyces cerevisiae, by comparing them with better known systems in both prokaryotes and eukaryotes. 

U Kompella, KJ Kim, MHI Shiue, VHL Lee. (1995). Possible existence of Na+-coupled amino acid transport in the pigmented rabbit conjunctiva. Life Sci 57 1427-1431. 

Arriza, J. L., Eliasof, S., Kavanaugh, M. P., and Amara, S. G. (1997) Excitatory amino acid transporter 5, a retinal glutamate transporter coupled to a chloride conductance. Proc. Natl. Acad. Sci USA 94,4155 1160. 

Molecular Neuropharmacology Strategies and Methods is intended to bridge the gap between molecular biology and advanced chemistry. In addition, it attempts to include information about x-ray crystallographic analyses whenever available. This book discusses interdisciplinary interactions for monoamine transporters, amino acid transporters, ionotropic receptors, metabotropic glutamate receptors, GABA receptors, and other G protein-coupled receptors. 

The next process is similar in both eukaryotes and prokaryotes, and involves the translation of mRNA molecules into polypeptides. This procedure involves many enzymes and two further types of RNA transfer RNA (tRNA) and ribosomal RNA (rRNA). There is a specific tRNA for each of the amino acids. These molecules are involved in the transportation and coupling of amino acids into the resulting... 

ATB Na+/Cl -coupled broad specificity amino acid transporter B0,+... 

T. Hatanaka, M. Haramura, Y. J. Fei, S. Miyauchi, C. C. Bridges, P. S. Ganapathy, S. B. Smith, V. Ganapathy, and M. E. Ganapathy. Transport of amino acid-based prodrugs by the Na" - and Cl -coupled amino acid transporter ATB° + and expression of the transporter in tissues amenable for drug delivery. J Pharmacol Exp Ther 308 1138-1147(2004)... 

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