Covalent attachment to proteins

The nonreactive base structures of cyanine dyes (or carbocyanines) have been used for many years as components in photographic emulsions to increase the range and sensitivity of film and also in CD-R and DVD-R optical disks to record digital information. A major innovation came when Ernst et al. (1989) and Waggoner et al. (1993) recognized that cyanine dyes would make excellent labels for fluorescence detection, and for this reason, they synthesized reactive dye derivatives, which then could be covalently attached to proteins and other molecules. 

We need to think about two issues. The first is the nature of the carbohydrate that gets added to proteins and the second is how it is attached. In general, carbohydrates covalently attached to proteins are oligosaccharides. Atypical oligosaccharide finked to a protein contains 5-12 monosaccharide units. 

More recently, and based on the same concept, O Neill and coworkers have prepared a series of related systems (80a-c) based on Vennerstrom s adamantyl trioxolane unit". These derivatives, which are synthesized in only three steps from adamantan-2-one, have activity in the low nanomolar region (<3 nM versus K1 P. falciparum). Like the prototypes synthesized by the groups of Meunier" and Singh", these compounds can be formulated as water-soluble salts and it is anticipated that these agents may have the capacity to hit the parasite by two distinctive mechanisms. Indeed, any chemical or metabolic degradation of the endoperoxide bridge in these compounds will result in metabolites that may still have the ability to function as inhibitors of heme polymerization provided that they do not become covalently attached to proteins in the bioactivation process. 

All of the glycosaminoglycans, except hyaluronic acid, are found covalently attached to protein, forming proteoglycan monomers. 

I Unsulfated, not covalently attached to protein, and only I GAG not limited to animal tissue, but also found in bacteria. 

While hyaluronan is not covalently attached to proteins, it is usually anchored to cell surfaces and to... 

As the final outer stratum comeum is formed the phospholipid bilayer deteriorates and intercellular lipid layers are formed.k l These contain principally ceramides, cholesterol, and free fatty acids. Some sphingolipids are covalently attached to proteins.3... 

One of the most rapidly advancing areas in biochemistry is that concerned with the structure and metabolism of oligosaccharides that are covalently attached to proteins (in glycoproteins) and lipids (in glycolipids). The carbohydrates in these complexes vary in composition, in the way they are linked, their branching patterns and in the sugars that terminate each branch. 

Limiting essential amino acids covalently attached to proteins by using activated amino acid derivatives can improve the nutritional quality and change the functional properties of proteins. The best chemical methods for incorporating amino acids into water-soluble proteins involve using car-bodiimides, N-hydroxysuccinimide esters of acylated amino acids, or N-carboxy-a-amino acid anhydrides. The last two methods can give up to 75% incorporation of the amount of amino acid derivative used. With the anhydride method, as many as 50 residues of methionine have been linked to the 12 lysine residues of casein. The newly formed peptide and isopeptide bonds are hydrolyzed readily by intestinal aminopeptidase, making the added amino acids and the lysine from the protein available nutritionally. 

Mossbauer labels covalently attached to proteins, presumably at the protein surface, exhibit temperature dependences similar to that described above for the heme iron and hydration dependences showing motion developing above a slightly higher hydration level than that found for the heme iron (Belonogova et ai, 1979 Likhtenshtein, 1976). 

Figure 12.26. Membrane Anchors. Membrane anchors are hydrophobic groups that are covalently attached to proteins (in blue) and tether the proteins to the membrane. The red and blue hexagons correspond to mannose and GlcNAc, respectively. R groups represent points of additional modification.

Fig. 1. Structures of lipids covalently attached to proteins. Panel A shows proteins that are lipidated on cytoplasmi-cally exposed amino acids, whereas panel B shows lipidated proteins in the extracellular leaflet. (A) iV-myristoyl glycine, palmitate thioester-linked to cysteine, farnesyl, or geranylgeranyl (prenyl) thioether-linked to cysteine. (B) A/-palmitoyl cysteine, cholesterol ester-linked to glycine, and a minimal GPI anchor linked to the to amino acid in a GPI-anchored protein. The GPI structure is shown with a diacylglycerol moiety containing two ester-linked fatty acids. Other GPI anchors are based on ceramide, while yet others have monoacylglycerol, a fatty acid ether-linked to glycerol, and/or a fatty acid ester-linked to inositol.

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