Linkage covalent

Like peptides, proteins are formed from amino acids through amide linkages. Covalently bound... 

The primary structure of a protein is the sequence of amino acids in its chain(s). Primary structure, which determines the other three kinds of structure, is maintained by the covalent peptide bonds between individual amino acids. The primary structure of egg-white lysozyme—a protein that helps fight infection—is shown in Figure 21.10 . The figure illustrates the amino acid sequence, the N-terminal and C-terminal ends, and the presence of disulfide linkages, covalent cross-links between cysteine amino acids in the polymer. We discuss disulfide linkages in more detail later in the section on tertiary structure. Researchers determined the first amino acid sequences for proteins in the 1950s. Today, the amino acid sequences for thousands of proteins are known. 

Glycoproteins or mucoproieins. Compounds of proteins with carbohydrates. All glycoproteins contain a hexosamine and usually sulphate, ethanoate and glucuronic acid. The carbohydrate-protein linkages are, in some cases covalent and in others of the salt type. Aqueous solutions of glycoproteins are extremely viscous. 

A completely dehydroxylated surface consists essentially of an array of oxygen atoms the Si-0 linkages are essentially covalent so that the silicon atoms are almost completely screened by the much larger oxygen atoms. Such a surface represents the extreme case and, even on samples ignited at 1100°C, a minute residue of isolated hydroxyl groups will be present. 

In contrast to monophosphates, starch phosphate diesters contain cross-links between two or more starch chains. This covalent linkage in the granule produces a starch product which swells less but is more resistant to heat, agitation, and acid than natural starch. 

Covalently Bound Flavins. The FAD prosthetic group in mammalian succinate dehydrogenase was found to be covalently affixed to protein at the 8 a-position through the linkage of 3-position of histidine (102,103). Since then, several covalently bound riboflavins (104,105) have been found successively from the en2ymes Hsted in Table 3. The biosynthetic mechanism, however, has not been clarified. 

The often fast binding step of the inhibitor I to the enzyme E, forming the enzyme inhibitor complex E-I, is followed by a rate-determining inactivation step to form a covalent bond. The evaluation of affinity labels is based on the fulfillment of the following criteria (/) irreversible, active site-directed inactivation of the enzyme upon the formation of a stable covalent linkage with the activated form of the inhibitor, (2) time- and concentration-dependent inactivation showing saturation kinetics, and (3) a binding stoichiometry of 1 1 of inhibitor to the enzyme s active site (34). 

The enzyme catalyzes the hydrolysis of an amide bond linkage with water via a covalent enzyme-inhibitor adduct. Benzoxazinones such as 2-ethoxy-4H-3,l-benzoxazin-4-one [41470-88-6] (23) have been shown to completely inactivate the enzyme in a competitive and stoichiometric fashion (Eigure 5). The intermediate (25) is relatively stable compared to the enzyme-substrate adduct due to the electron-donating properties of the ortho substituents. The complex (25) has a half-life of reactivation of 11 hours. 

These interactions are most commonly observed for divalent chalcogen atoms and the nitrogen atom (the electron donor D) lies within the X-E-Y (E = S, Se, Te) plane, preferably along the extension of one of the covalent bonds as in 15.3. This anisotropy is a clear indication that these short E N contacts have some bonding character, i.e., they are subject to the geometric restrictions of orbital overlap. Eor example, in the diselenide 15.4 the nitrogen lone pairs are clearly oriented towards the Se-Se linkage. ... 

Chemically, proteins are unbranched polymers of amino acids linked head to tail, from carboxyl group to amino group, through formation of covalent peptide bonds, a type of amide linkage (Figure 5.1). 

The peptide linkage is usually portrayed by a single bond between the carbonyl carbon and the amide nitrogen (Figure 5.3a). Therefore, in principle, rotation may occur about any covalent bond in the polypeptide backbone because all three kinds of bonds (N——C, and the —N peptide bond) are sin-... 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

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