Threonine acylation

A variety of proteins are acylated by formation of thioesters to cysteine and esters to serine and threonine. Acylation may serve either to anchor the proteins in membranes (e.g., rhodopsin Section 2.3.1) and the mannosidase of the Golgi, or to increase lipophilicity and thus enhance the solubilization of lipids being transported (e.g., the plasma apolipoproteins and milk globule proteins). Proteolipids with fatty acids esterified to threonine residues occur in the myelin sheath in nerves. 

Neuromotor disorders, including paresthesia of the hands and feet, hyperactive deep tendon reflexes and muscle weakness. These can be explained by the role of acetyl CoA in the synthesis of the neurotransmitter acetylcholine and the impaired formation of threonine acyl esters in myelin. Dysmyelination may explain the persistence and recurrence of neurological problems many years after nutritional rehabilitation in people who had suffered from burning foot syndrome. 

The site for attaching the acyl adenylate to tRNA is hydrophobic, so the valine acyl adenylate binds preferentially to that site. The site for hydrolyzing the acyl adenylate is polar, so the threonine acyl adenylate binds to that site. Thus, if threonine is achvatedby the aminoacyl-tRNA synthetase for valine, it will be hydrolyzed rather than transferred to the tRNA. 

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. 

TAG-CH3 and TAG-CH2-, acyl chain terminal-CH3 and bulk (-CH2-)n groups, respectively, of fatty acids (predominantly triacylglycerols) associated with chylomicron- and very low-density lipoprotein (VLDL) Thr, threonine-CHs Val, valine-CHs. The asterisk In spectrum (b) denotes a radiolytically-generated 2.74 p.p.m. 

The symmetrical anhydride is less reactive and consequently more selective in its reactions than the O-acylisourea. Although the latter can acylate both N- and O-nucleophiles, the symmetrical anhydride will only acylate V-nuclcophilcs. This means that the hydroxyl groups of the side chains of serine, threonine, and tyrosine that have not been deprotonated are not acceptors of the acyl group of the symmetrical anhydride. An additional feature of this approach to carbodiimide-mediated reactions is that it avoids a possible side reaction between the carbodiimide and the iV-nucleophilc, which gives a trisubstituted guanidine [(C6HuN)2C=N-CHR5CO-... 

The phosphonium and carbenium salts are efficient reagents for activating and coupling A-alkoxycarbonylamino acids as well as peptide acids. However, the requirement for tertiary amine to effect the reaction has several implications. The base renders hydroxyl groups subject to acylation. Hence, the side chains of serine and threonine and any hydroxymethyl groups of a resin that have not been derivatized... 

The serine hydrolases, threonine hydrolases, and cysteine hydrolases, the attacking nucleophile of which is a serine or threonine OH group or a cysteine thiolate group, respectively, and which form an intermediate covalent complex (i. e., the acylated enzyme). Here, an activated H20 molecule enters the catalytic cycle in the second step, i.e., hydrolysis of the covalent intermediate to regenerate the enzyme. 

The acylated peptides (Myr)GCX-Bimane 31 a-e (X = G, L, R, T, V), which are found in certain nonreceptor tyrosine kinases and ct-subunits of several heterotrimeric G-proteins, were synthesized in solution using common solution-phase peptide synthesis with X-myristoylglycine as a building block. These model peptides were used for acylation studies with palmitoyl-CoA in phospholipid vesicles at physiological pH. For such uncatalyzed spontaneous reactions only a modest molar excess of acyl donor species (2.5 1) was necessary. Unprotected side chains of threonine or serine are not interfering with this S-acylation (Scheme 14). 

Peptides containing serine or threonine may undergo an N—>0 acyl shift upon exposure to strong acids (Scheme 40). 592,594 This reaction has been exploited in the structure elucidation of cyclosporin A, since the acid-catalyzed acyl shift with formation of an ester allowed its selective hydrolysis to the linear peptide for further stepwise degradation. 593 ... 

Figure 13.2 A possible specificity mechanism for the prevention of the misacylation of tRNAVal with threonine, (a) The hydrophobic acylation site discriminates against threonine, (b) The hydrolytic site specifically uses the binding energy of the hydroxyl of threonine for a binding or catalytic effect. The translocation may occur as illustrated via a 2 - — 3 -hydroxyl acyl transfer. [From A. R. Fersht and M. Kaethner, Biochemistry 15, 3342(1976).]...

Aliphatic hydroxyl groups cannot normally be selectively modified except in certain special cases such as the serine proteinases. In anhydrous formic acid, the A,O-acyl migration that occurs in strong sulfuric or phosphoric acid apparently does not occur. Instead there is formylation of the serine and threonine residues (208). Enzymically inactive aggregates are produced, but the reaction is reversed in aqueous solution at neutral pH and the activity returns. Josefsson reported the introduction of 29 formyl groups in RNase (209) as compared to the total of 25 Ser and Thr residues. This identification of reaction sites is not clear, however, since the number of formyl groups introduced into lysozyme far exceeded the Ser-Thr total. 

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