Membranes transport proteins and

Unconjugated bilirubin is taken into the hepatocytes by binding to membrane transport proteins and transported through the liver cells to the SER by proteins called ligandins. The SER is the location of a specific bilirubin-UDP-glucuronosyl transferase... 

This exploding basic knowledge will enable researchers to Identify new types of compounds that inhibit or activate just one of these membrane transport proteins and not its homologs. An important challenge, however is to understand the role of an Individual transport protein In each of the several tissues In which It Is expressed. As an example, a drug that inhibits a particular Ion channel In sensory neurons might be useful In treatment of chronic pain, but if this channel also Is expressed In certain areas of the brain, its inhibition may have serious undesIred actions ( side effects ). 

Volume 23 Genes and Membranes Transport Proteins and Receptors ... 

An additional feature in Fig. 2 worth noting is the amino-terminal 160 amino acids of mercuric reductase that lacked a fixed position in the crystal and therefore were not part of the solved structure. These contain the sequence that is homologous to MerP and postulated to be a mercurybinding domain. This region is drawn in Fig. 2 as an extension from the protein perhaps it functions like a baseball mitt that catches Hg from the membrane transport proteins and delivers Hg " to the carboxyl-terminal catalytic binding site, so that, as in the bucket brigade model above, Hg " is never found free within the cell. Mutant strains with the transport system but lacking the MerA detoxification enzyme are hypersensitive to mercury salts, as they accumulate Hg " but cannot get rid of it. After reduction by NADH (via FAD and the active site cysteine pair), metallic Hg is released... 

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