Amino acid-bonded transporter

At least six different Na -dependent amino acid carriers are located in the apical brush border membrane of the epithelial cells. These carriers have an overlapping specificity for different amino acids. One carrier preferentially transports neutral amino acids, another transports proline and hydroxyproline, a third preferentially transports acidic amino acids, and a fourth ffansports basic amino acids (lysine, arginine, the urea cycle intermediate ornithine) and cystine (two cysteine residues linked by a disulfide bond). In addition to these Na -dependent carriers, some amino acids are transported across the luminal membrane by facilitated tiansport carriers. Most amino acids are transported by more than one tiansport system. 

The only consistent difference observed between the effects of the two enzymes appears to be a smaller effect of chymotrypsin on electron-transport and phosphorylation. This difference may be due to a lack of availability of the specific amino acid bonds which chymotrypsin attacks. 

Pregnenolone is transported from the mitochondria to the ER, where a hydroxyl oxidation and migration of the double bond yield progesterone. Pregnenolone synthesis in the adrenal cortex is activated by adrenocorticotropic hormone (ACTH), a peptide of 39 amino acid residues secreted by the anterior pituitary gland. 

Many biological molecules, such as proteins, are large and have complex structures. Proteins are composed of a sequence of units, called amino acids, which are joined by a strong covalent bond known as a peptide bond. But proteins also fold up into a certain shape, which is vital to their function—a protein that loses its shape cannot fulfill its function as an enzyme or a transporter. The sequence of amino acids determines the shape 20 different amino acids are found in proteins, and each one has slightly different chemical and physical properties. For example, some amino acids are hydrophilic, readily interacting... 

The other major class of transporter protein is the carrier protein. A prototypic example of a carrier protein is the large neutral amino acid transporter. An important function of the LNAA transporter is to transport molecules across the blood-brain barrier. As discussed previously, most compounds cross the BBB by passive diffusion. However, the brain requires certain compounds that are incapable of freely diffusing across the BBB phenylalanine and glucose are two major examples of such compounds. The LNAA serves to carry phenylalanine across the BBB and into the central nervous system. Carrier proteins, such as the LNAA transporter, can be exploited in drug design. For example, highly polar molecules will not diffuse across the BBB. However, if the pharmacophore of this polar molecule is covalently bonded to another molecule which is a substrate for the LNAA, then it is possible that the pharmacophore will be delivered across the BBB by hitching a ride on the transported molecule. 

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