Modified amino acids in proteins

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. 

Amino Acid Classes Biologically Active Amino Acids Modified Amino Acids in Proteins Amino Acid Stereoisomers Titration of Amino Acids Amino Acid Reactions PEPTIDES PROTEINS... 

See also Proline, Collagen, Modified Amino Acids in Proteins, Glutamate as a Precursor of Other Amino Acids (from Chapter 21)... 

See also Aromatic Amino Acid Utilization, Modified Amino Acids in Proteins... 

The modification of amino acids in proteins and peptides by oxidative processes plays a major role in the development of disease and in aging (Halliwell and Gutteridge, 1989, 1990 Kim et al., 1985 Tabor and Richardson, 1987 Stadtman, 1992). Tissue damage through free radical oxidation is known to cause various cancers, neurological degenerative conditions, pulmonary problems, inflammation, cardiovascular disease, and a host of other problems. Oxidation of protein structures can alter activity, inhibit normal protein interactions, modify amino acid side chains, cleave peptide bonds, and even cause crosslinks to form between proteins. 

Proteins are highly complex, folded polypeptide chains consisting of at least 20 different amino acids that are strung together in unique sequences, which relate to structure and function. Particular amino acids in proteins may be further modified post-translationally to contain a wide variety of covalent modifications normally found in native proteins. The way in which a peptide chain is wrapped and folded governs each amino acid s relative exposure to the outside environment, but post-translational modifications also can obscure the protein surface from easy access to the solvent environment. 

Isolation of individual amino acids started about 1820 by 1904 all of the naturally occurring amino acids in proteins had been isolated except methionine (Mueller, 1922) and threonine (Rose, 1937). One of the earliest methods for the separation of amino acids was through the differential volatility of their methyl or ethyl esters (Emil Fischer, 1901). This approach led to the discovery of valine, proline, and hydroxyproline. [In the 1970s Fischer s method was modified for microanalysis of proteins, separating the amino acid esters by gas phase chromatography. Separation is now usually performed by hplc (high pressure liquid chromatography).]... 

Occurrence of Modified Amino Acids in E. coli Ribosomal Proteins"... 

Amino acid moieties of many proteins are enzymatically modified within the intracellular compartments following their incorporation into the polypeptide chains. For example, many asparagine residues in proteins of enkaryotic cells have carbohydrates attached to them, converting the protein into a glycoprotein. Several other post-translationa 1 modifications involving many of the 20 amino acids in proteins have been observed. Table 2.4 is a partial list of post-translational modifications found in mammalian proteins. 

This report describes the presence of significant amounts of e-N-acetyllysine in rpST and rbST, eukaryotic proteins expressed in a prokaryotic system. Initial work from our laboratory has also demonstrated the presence of this modified amino acid in two other recombinant eukaryotic proteins expressed in E. coli, bovine placental lactogen and human tissue factor pathway inhibitor (16). ESMS, amino acid sequencing and amino acid analyses were utilized to demonstrate the presence of e-N-acetyllysine in these two recombinant proteins. These data established that this modified amino acid is present in several distinct recombinant eukaryotic proteins expressed in E. coli. 

Extra attention was devoted to proteins and en7ymes, as they have been the traditional "bread and butter" of the nutritional biochemist. The amino acids that form proteins can be classified in several ways (1) relatively hydrophilic or lipophilic, (2) dispensable or indispensable (noncssentiai or essential), (3) glycogenic or ketogenic, and, hnally, (4) classical or modified. Only a minority of the amino acids in proteins are modified after incorporation into the polypeptide chain. 

Direct Determination of the Contents of Amino Acids in Proteins. Amino acid contents of proteins are presently determined, with few exceptions, on an acid hydrolysate of the protein. In some instances, however, a particular amino acid may be at least partially destroyed by the conditions for acid hydrolyses. In other instances, a particular derivative of an amino acid of the protein may be hydrolyzed by the acid condition to regenerate the original amino acid. An analysis of such a chemically modified protein would thus need to be done on the protein before acid hydrolysis. In still other instances, there may be a need to perform a large number of analyses of different proteins for a single amino acid. The availability of a simple, direct chemical method for determination of such a single amino acid would be desirable. 

In the genetic code, methionine is coded for by the codon AUG. This codon is called the start codon because methionine is the first amino acid used to build a protein chain. Methionine forms the so-called amino terminus of a protein. In prokaryotes, a modified form of methionine, formyl-methionine is used as the first (but not subsequent) amino acid in proteins. Formyl-methionine is carried by a modified tRNA from the tRNA that carries unmodified methionine. 

MW Crankshaw, GR Grant. Identification of modified PTH-amino acids in protein sequence analysis. Presented at the Association of Biomolecular Resource Facilities, 1993. 

Modified amino acids. In addition to the amino acids encoded by DMA that form the primary structure of proteins, many proteins contain specific amino acids that have been modified by phosphorylation, oxidation, carboxylation, or other reactions. When these reactions are enzyme-catalyzed, they are referred to as post-translational modifications. 

Fig. 2 Hapten/peptide and p-i concept of T cell stimulations by drugs, (a) Haptens haptens are chemically reactive compounds. Drugs like penicillin are haptens they bind to lysine amino acids in proteins or by the SH-group formed by opening the thiazolidine ring. This covalent binding can modify soluble or cell bound molecules. They can even bind directly to the immunogenic major histocompatibiliy complex (MHC)/peptide complex on antigen presenting cells (ARC), either to the embedded peptide or to the MHC molecule itself. Thus, the chemical reactivity of haptens can lead to...

With individual amino acids in proteins. This results in a chemical modification of the protein, which may therefore be recognized as foreign by the immune system, leading to the production of antibodies against the modified protein that will also react with the normal, unmodified body protein. This may be an important factor in the development of autoimmune disease. 

With unsaturated fatty acids in cell membranes. This leads to the formation of dialdehydes, which react with DNA, causing chemical modification, and hence may result in either heritable mutations or initiation of cancer. Dialdehydes can also react with amino acids in proteins, leading to modified proteins that stimulate the production of autoantibodies. 

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