Serine cluster

Differently from serine, ESI-MS analysis of homoserine (HSer) solutions reveals an unusually abundant diprotonated homoserine octamers [(HSer)g-2H], but not the expected monoprotonated [(HSer)g H]" one." A 3/1 mixture of L-serine and L-homoserine yields abundant mixed serine octamers with the incorporation of one or two homoserine molecules into the cluster. CID of the isolated [(Ser)6(HSer)2-H] cluster leads to the preferentially loss of two neutral serine molecules. Homoserine is always retained. The ESl-MS spectral patterns of threonine and allothreonine solutions is similar to that of homoserine. A 1/1 mixture of D-serine and D-threonine yields abundant mixed singly- and doubly-charged octamers incorporating from 2 to 6 threonine molecules. Their relative abundance indicates that threonine may incorporate freely into serine clusters because the additional methyl group does not interfere with the bonding of the cluster. 

Schalley and Weis [98] revised the results obtained by several studies on hydrogen bonded serine dimers. Those have reported the formation of unusually stable protonated serine octamers, with a clear preference for homochirality [99-103]. Mixed l- and D-serine clusters show up in a small ratio vs. the homochiral ones. 

The Synapt has been used to investigate the conformational structure of the serine octomer [105] this is an interesting experiment because this octomer is related to the development of homochirality. Serine clusters composed of eight amino-acids have been found to have a remarkable preference for homochirality. Mobility MS and MS/ MS experiments were carried out and demonstrated that two major isomeric forms of the molecule are present the more compact form is able to dissociate readily to the protonated dimer and serine monomer, whilst the less compact form is much more resistant to dissociation. These results are to be compared with theoretical calculations to throw new light on the structure of these potentially very important molecules. 

Lapko, V.M., Jiang, X.-Y, Smith, D.L., and Song, P.-S., Posttranslational modification of oat phytochrome A phosphorylation of a specific serine in a multiple serine cluster. Biochemistry, 36,... 

Early mutational studies of the Rieske protein from 6ci complexes have been performed with the intention of identifying the ligands of the Rieske cluster. These studies have shown that the four conserved cysteine residues as well as the two conserved histidine residues are essential for the insertion of the [2Fe-2S] cluster (44, 45). Small amounts of a Rieske cluster with altered properties were obtained in Rhodobacter capsulatus when the second cysteine in the cluster binding loop II (Cys 155, corresponding to Cys 160 in the bovine ISF) was replaced by serine (45). The fact that all four cysteine residues are essential in Rieske clusters from be complexes, but that only two cysteines are conserved in Rieske-type clusters, led to the suggestion that the Rieske protein may contain a disulfide bridge the disulfide bridge was finally shown to exist in the X-ray structure (9). 

The trinuclear cluster is surrounded by four pairs of histidine residues with Cu-N distances of approximately 2.1(1) A (Figure 5). Domain 1 donates two pairs H101, H103 and H161, H163 with a serine residue... 

One may use the stronger term chirality discrimination when a substantial suppression of one intermolecular diastereomer with respect to the other occurs. This requires multiple strong interactions between the two molecular units and therefore more than simple monofunctional alcohols. Some examples where one of the molecules involved is a chiral alkanol are reported in Refs. 112 and 119 121. Pronounced cases of higher-order chirality discrimination have been observed in clusters of hydroxy esters such as methyl lactate tetramers [122] and in protonated serine octamers [15,123,124]. The presence of an alcohol functionality appears to be favorable for accentuated chirality discrimination phenomena even in these complex systems [113,123,125,126]. Because the border between chirality recognition and discrimination is quite undefined, it is suggested that the two may be used synonymously whenever both molecular partners are permanently chiral [127]. 

U. Mazurek, Some more aspects of formation and stability of the protonated serine octamer cluster. Eur. J. Mass Spectrom. 12, 63 70 (2006). 

The precursor for the class III lantibiotic SapB contains a 21-amino-acid leader peptide. Two ABC-transporters RamA and RamB, consisting of 636 and 608 amino acids, respectively, are encoded by the SapB gene cluster. RamA and RamB share 31 % sequence identity however, neither contains a serine protease or cysteine protease domain. Since no other candidate proteases are present in the gene cluster, the identity of the protease that removes the leader peptide remains elusive. 

Figure 5.1. Structures, oxidation states and spin states of crystallographically defined Fe-S clusters. Complete cysteinyl ligand (as shown) is most common, although a limited number of examples are known in which histidine, serine, and aspartate residues can act as cluster ligands. The spin states denoted by a question mark have yet to be determined, and the [FesSJ cluster has been observed only as a fragment in heterometaUic [MFejSJ clusters in which M is a divalent transition metal ion.

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