The Hydroxyamino Acids

Phosphorylation is the reversible process of introducing a phosphate group onto a protein. Phosphorylation occurs on the hydroxyamino acids serine and threonine or on tyrosine residues targeted by Ser/Thr kinases and tyrosine kinases respectively. Dephosphorylation is catalyzed by phosphatases. Phosphorylation is a key mechanism for rapid posttranslational modulation of protein function. It is widely exploited in cellular processes to control various aspects of cell signaling, cell proliferation, cell differentiation, cell survival, cell metabolism, cell motility, and gene transcription. 

Hymenocardine (58), (32, 40) has a p-hydroxy-w-aminoacetophenone unit (81), instead of the usual styrylamine, in its cyclic system which can be recognized in addition to N,N-dimethylisoleucylvaline and tryptophan in its acid hydrolysate. Mild alkaline hydrolysis results in ring opening via /3 elimination on the hydroxyamino acid and severance of the phenolate to a tetrapeptide 82 whose structure was determined by mass spectrometry and further hydrolysis. It is the only peptide alkaloid in which /3-hydroxy valine is involved in the aryl ether bridge. 

Frequently the configuration of the hydroxyamino acid component of the peptide alkaloid could be deduced with the aid of NMR studies. The coupling constants, JaB, of several a-amino-j8-hydroxyamino acids as well as their A,iV-dimethyl derivatives and their methyl ethers were determined by Marchand et al. (34). They showed that only the N,N-dimethyl derivatives yielded JaB values of configurational significance and that the conversion of the hydroxyl to alkoxyl exerted virtually no influence. The Ja0 value of Ar,A-dimethyl-j3(p-tolyloxy)leucine in the threo form was 8.5 Hz whereas that in the erythro form was 2.5 Hz. Wenkert et al. studied frangulanine (10), discarine-A (6), and discarine-B (7) whose amide protons had been replaced by deuterium (22). The a- and jS-methine signals of hydroxyleucine occurred at 84,40 and 4.77, respectively, as a doublet with JaB = 8 Hz and as a double... 

Procedures have thus been provided for the determination of serine and threonine in protein hydrolysates l4-l7v74-81 and of the hydroxyamino acids of insulin.1 ... 

L-serine. The latter three fragments arise by slow decomposition of a primary unit of 2-(2-hydroxy-6-methylphenyl)oxazoline. Acid hydrolysis also liberates /ra s-octadec-2-enoic acid, but the cis isomer is obtained by periodate cleavage and hence represents the natural configuration. The length of the n-A2-unsaturated fatty acid side chain ranges from C14 to C20 with Cig representing the main component (85%). No method has been found to separate mycobactins P with different side chains. Saponification yields mycobactic acid P and a neutral substance, cobactin P. Reductive hydrolysis in HI gives the hydroxyamino acid as L-lysine. 

A particularly striking feature of the simplified formula of the tobacco mosaic virus (Fig. 14) is the stereotype repetition of several simple structures (combinations UI, LS, AF) which in fact represent the cornerstones of the entire molecule. A phenomenon deserving special interest (apart from a number of symmetrical sequences occurring along the whole chain) is the symmetrical arrangement of the dipeptides of the hydroxyamino acid residues at both terminals of the polypeptide chain. This relative concentration of the serine residues at the ends of the tobacco mosaic virus protein chain has been pointed out by Anderer et al. (1960) as a possible example of nonrandomness. 

Spectra of many of the common amino acids not yet discussed are shown in Fig. 2, kindly made available by Sussman and Gratzer (1962). The data of this figure are usefully supplemented by the earlier but less extensive (2000-2300 A) observations of Saidel et al. (1952). In addition to some duplication of the amino acids of Fig. 2, Saidel et al. showed spectra of cysteic acid, hydroxyproline, asparagine, valine, isoleucine, and glycine. The monoamino, monocarboxylic amino acids, including the hydroxyamino acids and the imino acids, are seen to have very similar spectra at wave-... 

Apart from His, Cys and Ser, the hydroxyamino acid threonine can also be a residue in the active site of phosphatases the Thr214Ala mutant of Na+K+-ATPase exhibits a drastically reduced vanadate affinity for inhibition of ATPase activity,I ] suggesting that Thr is essential in vanadate binding. In the light of what has been said above, threonine very probably coordinates to vanadium into one of the axial positions in much the same manner as the other amino acid residues. 

The biosynthesis of bufotenine has not been entirely elucidated experimentally. Some biochemical observations, however, prompted Witkop (141) even at early stages of investigation to make the accepted (157) suggestion of the possibility of direct hydroxylation of the benzene ring in tryptophan prior to any reaction on the pyrrole part of the molecule. Since the hydroxyamino acid was found not only in toad secretions but later also in the mushrooms, it may be the precursor of serotonin and hence A/ -dimethyl derivative, bufotenine. 5-Hydroxytryptophan was first found... 

For complete hydrolysis of bonds between hindered amino acid residues the above mentioned 16 hour period is not sufficient. In addition to isoleucine and valine synthetic intermediates can contain residues with bulky and acid resistant blocking groups, such as S-benzylcystein. Should the latter be preceded or followed by a hindered amino acid, then the time of hydrolysis must be considerably prolonged. Since the hydroxyamino acids and also tryptophan gradually decompose under these conditions, it might be necessary to carry out more than one analysis, with shorter and longer hydrolysis times. 

Hamada and Shioiri have subsequently demonstrated the utility of this methodology through the synthesis of a range of natural products including prumycin, L-daunosamine vide infra), mugineic acid, and the hydroxyamino acid moiety of AI-77-B. ... 

D-Ribono- and D-gulonolactones have been converted into hontochiral 3-hydroxymethyl morpholinium derivatives 186 while the D-ribose-daived lactone 187 has been used to synthesize the pyrrolidinone 188, a precursor to the hydroxyamino acid moiety of AI-77B.79L-Quebrachitol has been converted to the lactam 189, a component of bengamide... 

The N O acyl rearrangement reaction at the hydroxyamino-acid residues (Ser and Thr) and the selective chemical cleavage of the chain at the resulting ester bonds have been studied in detail using unfractionated clupeine (Iwai, 1959, 1960, 1961 Iwai and Ando, 1967). This method of degradation retains intact sequences of arginine residues in the products. 

Gruen (757) found that the presence of most of the amino acids tested during hydrolysis does not affect the tryptophan recovery. Of the hydroxyamino acids, threonine had no effect at all, while tyrosine has a small effect. On the other hand serine was found to reduce the recovery of tryptophan by about 10% due to a deamination reaction of serine. This produces pyruvic acid which, as a ketoacid, interacts with the indole nucleus of tryptophan (287). Whereas cysteine had no effect, cystine caused a substantial loss of tryptophan (about 40%), and was itself recovered on the analyzer partly in the reduced cysteine form. These results would indicate that a major factor in the loss of tryptophan during acid hydrolysis of a protein is degradative oxidation by cystine. 

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