Serine residues modification

The serine residue of isocitrate dehydrogenase that is phos-phorylated by protein kinase lies within the active site of the enzyme. This situation contrasts with most other examples of covalent modification by protein phosphorylation, where the phosphorylation occurs at a site remote from the active site. What direct effect do you think such active-site phosphorylation might have on the catalytic activity of isocitrate dehydrogenase (See Barford, D., 1991. Molecular mechanisms for the control of enzymic activity by protein phosphorylation. Bioehimiea et Biophysiea Acta 1133 55-62.)... 

Control of pymvate dehydrogenase activity is via covalent modification a specific kinase causes inactivation of the PDH by phosphorylation of three serine residues located in the pyruvate decarboxylase/dehydrogenase component whilst a phosphatase activates PDH by removing the phosphates. The kinase and phosphatase enzymes are non-covalently associated with the transacetylase unit of the complex. Here again we have an example of simultaneous but opposite control of enzyme activity, that is, reciprocal regulation. 

Furthermore, post-translational modifications activate steroid hormone receptors in a ligand-independent fashion (Fig. 5), as shown for the ERa which is phosphorylated on serine residue 118 in the AF-1 domain by the Erkl/2 kinase [71]. In vitro, the serine-118 phosphorylated ERa is transcriptionally active in a ligand-independent fashion. 

The answer is D. Organophosphates react with the active site serine residue of hydrolases such as acetylcholinesterase and form a stable phosphoester modification of that serine that inactivates the enzyme toward substrate. Inhibition of acetylcholinesterase causes overstimulation of the end organs regulated by those nerves. The symptoms manifested by this patient reflect such neurologic effects resulting from the inhalation or skin absorption of the pesticide diazinon. 

Selective chemical change of the serine—OH group to cysteine—SH in enzymes can be performed with extremely reactive serine residues in the active sites by the use of phenylmethylsulfonyl fluoride and, subsequently, thioacetic acid (Polgar and Bender, 1966). This selective chemical modification demonstrates the essential role of an—OH... 

Aspirin is the only known NSAID that covalently bonds to serine and inhibits COX-1 more significantly than COX-2. Many systematic structural modifications have been carried out resulting in the development of APHS characterized by a 60-fold increase in activity and a 100-fold increase in selectivity for COX-2 than aspirin. Inhibition of COX-2 also occurs by acetylation of the same serine residue that is acetylated by aspirin, indicating that the mechanism of APHS inhibition is not identical to that of other selective COX-2 inhibitors (Kalgutkar et al., 1998a 1998b). 

The greatly increased nucleophilicity of the catalytic serine distinguishes it from all other serine residues and makes it an ideal candidate for modification via activity-based probes [58]. Of the electrophilic probe types to profile serine hydrolases, the fluorophosphonate (FP)-based probes are the most extensively used and were first introduced by Cravatt and coworkers [38, 39]. FPs have been well-known inhibitors of serine hydrolases for over 80 years and were first applied as chemical weapons as potent acetylcholine esterase inhibitors. As FPs do not resemble a peptide or ester substrate, they are nonselective towards a particular serine hydrolase, thus allowing the entire family to be profiled. FPs also show minimal cross-reactivity with other classes of hydrolases such as cysteine-, metallo-, and aspartylhydrolases [59]. Furthermore, FP-based probes react only with the active serine hydrolase, and not the inactive zymogen, allowing these probes to interact only with functional species within the proteome [59]. Extensive use of this probe family has demonstrated their remarkable selectivity for serine hydrolases and resulted in the identification of over 100 distinct serine hydrolases... 

Hydroxyproline and hydroxylysine occur most noticeably in collagen. These are formed by modification of proline and lysine residues by specific enzymes after synthesis of the collagen chains. It is interesting to note that proly/hydroxylase, which hydroxylates proline, requires ascorbate (vitamin C) as a coreactant. Other chemical modifications known to occur commonly are the attachment of sugars (glycosylation) to asparagine, serine, and threonine residues and the phosphorylation of serine. Chemical modifications are also associated with the transport of proteins out of the cells in which they are synthesized. 

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. 

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. 

For example, modifications outside the Q/j domain, such as phosphorylation of ataxin-1 at a crucial serine residue (Emamian et al., 2003) and SUMOylation of Htt (Steffan et al., 2004) are important determinants of toxicity. Moreover, Boat (brother of ataxin-1) was shown to interact with ataxin-1 at multiple sites, and altered expression of Boat in Purkinje cells may contribute to the neurodegeneration in SCAl (Mizutani et al., 2005). Differences in susceptibility to TG-catalyzed cross-linking among the various mutated Q -containing proteins may also contribute to the selectivity. 

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