Postsynthetic modification

Vitamin K Is the Coenzyme for Carboxylation of Glutamate in the Postsynthetic Modification of Calcium-Binding Proteins... 

A number of iron-containing, ascorbate-requiring hydroxylases share a common reaction mechanism in which hydroxylation of the substrate is linked to decarboxylation of a-ketoglutarate (Figure 28-11). Many of these enzymes are involved in the modification of precursor proteins. Proline and lysine hydroxylases are required for the postsynthetic modification of procollagen to collagen, and prohne hydroxylase is also required in formation of osteocalcin and the Clq component of complement. Aspartate P-hydroxylase is required for the postsynthetic modification of the precursor of protein C, the vitamin K-dependent protease which hydrolyzes activated factor V in the blood clotting cascade. TrimethyUysine and y-butyrobetaine hydroxylases are required for the synthesis of carnitine. 

The similarity of the various histone fibers is probably correlated with the similarity in the distribution of the amino acids in the sequences of the four core histones and reflects their function as the skeleton or backbone of chromatin. However, from the presence of a specific pattern of interactions of the core histones and the existence of histone variants and histone postsynthetic modifications, one can anticipate modulations in the basic general pattern of histone structure. In Section V, a possible mechanism for histone microheterogeneity influencing chromatin structure is suggested. Analogous to other assembly systems, small subunit modifications may be amplified to produce major changes in the assembled superstructure. 

Sterner, R., Vidali, G., Heituikson, R.L., and Allfrey, V.G. (1978) Postsynthetic modification of high mobility group proteins. Evidence that high mobility group proteins are acetylated. J. Biol. Chem. 253, 7601-7604. 

As a rule, these compounds were obtained using phenols, hydroxy-quinolines, and other hydroxy-substituted heterocycles with UNs. In some cases aminochromenes were subjected to postsynthetic modifications (05BML4745, 07JME417) (Scheme 56). 

Selenocysteine is a special case. This rare amino acid residue is introduced during protein synthesis rather than created through a postsynthetic modification. It contains selenium rather than the sulfur of cysteine. Actually derived from serine, selenocysteine is a constituent of just a few known proteins. 

A quite different situation holds for collagen in which P-galactosyl units and glucosyl-P-galactosyl disaccharide units are attached to side chains of hydroxy-lysine formed by postsynthetic modification of the original procollagen chain. 

Postsynthetic modifications of cytoskeletal microfilaments can also occur. For example, epidermal keratin has been found to contain lanthionine, (y-gluta-myllysine) and lysinoalanine, both presumably arising from crosslinkages.306... 

The labeling must be specific, that is, directed to a specific site in the RNA to yield meaningful results pertaining to specific nucleotides. This is commonly referred to as site-directed spin-labeling (SDSL) (Altenbach et ah, 1989 Barhate et ah, 2007 Edwards et ah, 2001 Kim et ah, 2004 Qin et ah, 2001, 2003 Schiemann et ah, 2004). Therefore, incorporation of multiple labels through enzymatic RNA synthesis (e.g., triphosphate polymerization with polymerases Keyes et ah, 1997) is of limited value. Instead, labels are generally introduced chemically, either during chemical synthesis of the nucleic acid or by postsynthetic modification of the polymer. 

Figure 15.3 Nucleotides that have been spin-labeled by postsynthetic modification of the oligomer at either the base (A), the sugar (B), the internal phosphodiester (C), or the terminus (D). B indicates the nucleoside base.

Scheme 8. Postsynthetic modification of RNA by using the convertible nucleosides shown in Scheme 7.

The second route to postsynthetic modification of SAMs is the chemical transformation of functional groups present on their outermost surface this approach mostly relies on chemistries already established for the functionalization of solid supports (Fig. 4.3). Two important points to bear in mind are (1) it is essentially impossible to extensively characterize the structure of the reaction products or purify them without destroying the SAM and (2) many solution-phase reactions may be very difficult when carried out on a surface because of the steric hindrance due to the very closely packed end groups. 

Clearly, these considerations imply that the preparation of functionalized SAMs via postsynthetic procedures requires reactions proceeding under mild conditions compatible with the structure and the integrity of the self-assembled monolayer. In addition, these reactions should, ideally, proceed quantitatively these considerations actually limit the reactions commonly used for SAM functionalization to a handful. In fact, the number of reactions used for postsynthetic modifications of SAMs is relatively small, and is essentially limited to acylation reactions (either amidations... 

One of the most attractive features of monolayer-protected AuNPs is the possibility for facile introduction of a wide variety of different thiolated molecules into the ligand shell. This may be achieved using different strategies (1) direct synthetic methods,97 124 170 (2) the so-called place-exchange reaction, (3) postsynthetic modifications, or (4) displacement of weak ligands (or protective agents) by functional ligands. 

The peptide bond between glutamic acid and cysteine is through the y-carboxyl of glutamic acid, not the a-carboxyl group. All natural proteins are composed with a-peptide links (unless there have been postsynthetic modifications). 

The most smdied enzyme is histidine decarboxylase from Lactobacillus 30a. There are pyruvate residues at the amino terminals of each of 5 of the 10 subunits in this enzyme. When the organism is grown on [ C] serine, the specific radioactivity of the pymvate is the same as that of serine incorporated into the protein and much greater than that of free lactate or pyruvate in the culture medium This suggests that pyruvate arises by postsynthetic modification of a serine residue. 

Topaquinone and lysyltopaquinone are formed by postsynthetic modification of the precursor proteins of the active enzymes. A tyrosine residue in the enzyme undergoes autocatalytic oxidation in the presence of enzyme-bound copper and oxygen. Lysyltopaquinone in lysyl oxidase is believed to be synthesized by reaction between topaquinone and the e-amino group of a lysine residue to form the cross-linked imino adduct. This means that it is highly improbable that either topaquinone or lysyltopaquinone is a dietary essential, because there is no way in which preformed quinone could be incorporated into the precursor protein or a hypothetical apoenzyme. 

Biotin is bound covalently to enzymes by a peptide fink to the s -amino group of a lysine residue, forrningbiotinyl-s-arnino-lysine orbiocytin (see Figure 11.1). This postsynthetic modification is catalyzed by holocarboxylase synthetase with the intermediate formation of biotinyl-5 -AMP. In bacteria, this intermediate also acts as a potent repressor of all four enzymes of biotin synthesis. 

Aspartate undergoes /3-decarboxylation to /S-alanine unlike most amino acid decarboxylases, aspartate decarboxylase is not pyridoxal phosphate-dependent, but has a catalytic pyruvate residue, derived by postsynthetic modification of a serine residue (Section 9.8.1). Pantothenic acid results from the formation of a peptide bond between /3-alanine and pantoic acid. 

In addition to its coenzyme role in postsynthetic modification of collagen and other connective tissue proteins, there is evidence that vitamin C is involved in the regulation of connective tissue protein gene expression (Mahmoodian and Peterkofsky, 1999). The expression of a number of other genes has also been reported to be modulated by vitamin C, including... 

More than half of the peptide hormones undergo postsynthetic modification to form a carboxy terminal amide, which is essential for biological activity. One function of this amidation is to render the peptides more hydrophobic and enhance receptor binding. The amide group is derived from a glycine residue that is to the carboxyl side of the amino acid which will become the amidated terminal of the mature peptide. 

Proline and lysine hydroxylases are found in the lumen of rough endoplasmic reticulum. Hydroxylation of the peptide substrate occurs both cotransla-tionaUy and later as a postsynthetic modification. The enzymes act only on peptides and not on free amino acids. 

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