Insulin hydroxylated

Another zinc-utilizing enzyme is carbonate/dehydratase C (Kannan et al., 1972). Here, the zinc is firmly bound by three histidyl side chains and a water molecule or a hydroxyl ion (Fig. 27). The coordination is that of a distorted tetrahedron. Metals such as Cu(II), Co(Il), and Mn(ll) bind at the same site as zinc. Hg(II) also binds near, but not precisely at, this site (Kannan et al., 1972). Horse liver alcohol dehydrogenase (Schneider et al., 1983) contains two zinc sites, one catalytic and one noncatalytic. X-Ray studies showed that the catalytic Zn(II), bound tetrahedrally to two cysteines, one histidine, and water (or hydroxyl), can be replaced by Co(II) and that the tetrahedral geometry is maintained. This is also true with Ni(Il). Insulin also binds zinc (Adams etai, 1969 Bordas etal., 1983) and forms rhombohedral 2Zn insulin crystals. The coordination of the zinc consists of three symmetry-related histidines (from BIO) and three symmetry-related water molecules. These give an octahedral complex... 

A group of receptors exists that responds to so-called growth factors such as insulin, epidermal growth factor, platelet-derived growth factor, etc. These receptors have an extracellular domain that binds the growth factor and an intracellular domain that possesses latent kinase activity. The interaction of insulin, for example, results in autophosphorylation of the intracellular domain and subsequent internalization of the insulin-receptor complex. The internalized complex now possesses the properties of a tyrosine kinase and can phosphorylate cell substrates that produce the appropriate intracellular effect. However, these kinases differ from the usual protein kinases in that they phosphorylate proteins exclusively on tyrosine hydroxyl residues. The ensemble of proteins phosphorylated by the insulin receptor has not yet been identified, but there is supportive evidence that tyrosine kinase activity is required for the major actions of insulin. For example, it is possible that a membrane-linked glucose transport system becomes activated following insulin-stimulated phosphorylation. 

In another approach, insulin was modified to introduce hydroxyl groups so that the hydroxylated insulin can be immobilized by forming a complex with phenylboronic acid groups on the support (Fig. 16.11). The support can be hydrogel beads made of polymers containing phenylboronic acid, e.g. poly(m-methacrylamidophenylboronic acid). The hydroxylated insulin can be displaced by the added glucose and the displaced insulin can be released. 

The enhancement of nasal absorption of insulin by hydrophobic bile salts has also been investigated. It was found that minor differences in the number, position, and orientation of the nuclear hydroxyl groups as well as alterations to side-chain conjugation can improve the adjuvant potency of bile salts. Moreover, the absorption of insulin positively correlated with an increase in the hydrophilicity of the steroid nucleus of the bile salts. In the presence of bile salts, nasal absorption of insulin reached peak levels within about lOmin, and some 10-20% of the dose was found to have been absorbed into the circulation. Marked increases in serum insulin levels were seen with sodium deoxycholate, the most lipophilic of the bile salts, whereas the least elevation—as well as least lowering of blood glucose levels—was seen with the most hydrophobic bile salt, sodium ursodeoxycholate [63],... 

Beyond translation, proteins and peptides may be further modified by metabolic events within the cell. For instance, the hydroxyproline and hydroxylysine in collagen are formed from proltne and lysine that are hydroxyl-ated while they are part of the precursor protein molecule. A protein may also be post-translationally modified by cleavage of select bonds and/or by addition or subtraction of various kinds of groups. Insulin, for instance, is formed cleavage of a larger protnsultn molecule. Similar events occur in the activation of zymogens. 

Many hormones, such as epinephrine (adrenaline), alter the activities of enzymes by stimulating the phosphorylation of the hydroxyl amino acids serine and threonine phosphoserine and phosphothreonine are the most ubiquitous modified amino acids in proteins. Growth factors such as insulin act by triggering the phosphorylation of the hydroxyl group of tyrosine residues to form phosphotyrosine. The phosphoryl groups on these three modified amino acids are readily removed thus they are able to act as reversible switches in regulating cellular processes. The roles of phosphorylation in signal transduction will be discussed extensively in Chapter 14. 

Each p subunit consists primarily of a protein kinase domain, homologous to protein kinase A. This kinase differs from protein kinase A in two important ways. First, the insulin receptor kinase is a tyrosine kinase that is, it catalyzes the transfer of a phosphoryl group from ATP to the hydroxyl group of tyrosine, rather than serine or threonine, as is the case for protein kinase A. 

These results support the hypothesis that superoxide or other oxygen radicals derived from it such as singlet oxygen [444] and hydroperoxyl or hydroxyl radicals may mediate 8-cell damage and the decrease in insulin, resulting in insulin-dependent diabetes [435]. 

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