Synthesis, Characterization, and Racemization of Poly-L-serine<sup>*</sup>

Bohak, Zvi; Katchalski, Ephraim

doi:10.1021/bi00902a005

Cited by 69 publications

(34 citation statements)

References 47 publications

(34 reference statements)

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“…It is interesting that the molecule is notably rich in serine and threonine, for it appears that polyserine [49,50], and indeed silk fibroin [48,51], which is rich in serine, are particularly prone to form p-structures.…”

Section: Discussionmentioning

confidence: 99%

A Conformational Study of Glucagon

Gratzer

Beven

Rrattle

et al. 1968

European Journal of Biochemistry

106

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Glucagon is a polypeptide of 29 residues, which can be crystallised. Optical rotatory dispersion in dilute neutral, acid and alkaline solution suggests that the polypeptide chain is largely random, but contains about one turn of a-helix, which is eliminated in 6 M guanidine hydrochloride.Using thermal difference spectra, nuclear magnetic resonance, and the temperature-dependence of the optical rotatory dispersion as criteria, it appears that glucagon possesses no defined tertiary structure in solution, but may be regarded as bcing in a state ofa-helix s random coil equilibrium, near the high-temperature end of the transition range. That there is no steric objection to the formation of a highly a-helical conformation is demonstrated by optical rotatory dispersion measurements in 2-chloroethanoi solution, in which the protein becomes largely helical. A rough calculation indicates that in aqueous solution a t pH 2 the transition mid-point is at about -40". From changes in the proton magnetic resonance spectrum when glucagon is transferred from an aqueous environment into 6 M guanidine hydrochloride, it is deduced that the short helical segment in the chain in the aqueous state contains most of the long aliphatic side chains, and since the sequence is known it has been possible to identify such a segment uniquely near the C-terminal end. On standing in acid solution the viscosity increases and a birefringent gel is formed. Sedimentation studies indicate the formation of large aggregates. On further standing, or warming, a precipitate appears which has the appearance of long fibrils in the electron microscope. Infrared spectra of the gel, of solid films and of the precipitated material show that in all these states the glucagon is in the form of antiparallel P-chains.Glucagon is a peptide hormone of molecular weight 3,500, with a known sequence [I]. It has been reported [21 that glucagon crystals are cubic, and it was suggested that the molecule in the crystal is cylindrical, and t,herefore in the a-helical form. The only molecules of comparable length which are known to exist in the fully helical state are homopolypeptides, and a considerable body of evidence has built up [3,4] which indicates that the helical segments in globular proteins depend for their conformational stability on hydrophobic contacts in the interior of the protein. Moreover no proteins in the size range of glucagon have so far been found to have rigid tertiary structures. It is of interest t o investigate the secondary structure of glucagon under various conditions, in order to determine the nature of the secondary structure, and whether a tertiary structure can be said to exist. A brief account of part of the work on the aggregated states has appeared [5]. EXPERIMENTAL PROCEDURE MaterialsThe glucagon samples used in this study were given by Dr. J. Schlichtkrull (Novo Research Institute) and Mr. T. Couling (Lilly Research Laboratories Ltd.). This crystalline material was homogeneous by paper electrophoresis a t several pH values. I n starch gel ...

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Section: Discussionmentioning

confidence: 99%

A Conformational Study of Glucagon

Gratzer

Beven

Rrattle

et al. 1968

European Journal of Biochemistry

106

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“…Certain amino acid derivatives such as N -acylamino acid amides may be good model compounds for studying the variables which affect the racemization of peptide bound amino acids [25]. That this is feasible was demonstrated [26] by the observation that the rate of racemization of N-acyl-L-serine amide in O.2M NaOH at 24°C was found to be comparable to the racemization rate of seryl residues of polyserine. There have been no investigations of the racemization rates of N -acylamino acid amides in the neutral pH region, however.…”

Section: Amino Acid Deriva Tlvesmentioning

confidence: 99%

Racemization of Amino Acids

Bada¹

1985

Chemistry and Biochemistry of the Amino Acids

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A century ago it was first observed that amino acids underwent racemization when heated in strongly acidic and basic solutions. It soon became well established that the optically active amino acids isolated from biological materials could be converted into a racemic mixture by a variety of rather vigorous treatments. In the early part of the 20th century the first observations of amino acid racemization in peptides and proteins in alkaline solutions at elevated temperatures were reported. The interpretation of these measurements was complicated by peptide-bond hydrolysis and the realization that racemization rates probably depended on whether amino acids were at terminal or internal positions in the peptide. The earlier work on amino acid racemization in various systems was extensively reviewed in 1948 by Neuberger [I].While racemization of amino acids at extreme pH values and elevated temperatures has been known for 100 years it has onl y been within the last 10-12 years that it was shown that racemization also takes place at neutral pH at rates which are comparable with those in dilute acid and base [2,3]. Also racemization was detected in fossils and the metabolically stable proteins in living mammals, and it was suggested that racemization might form the basis of a dating method which could be used to determine the age of amino acids in these systems [4][5][6][7][8].Racemization is a reversible first order reaction which can be written aswhere k j is the first order rate constant for the interconversion of amino acid enantiomers. The kinetic equation [4,5] for this reaction iswhere (

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“…Equation 1, as it stands, is not readily amenable to graphical analysis, however, it can be closely approximated by polymers poly-L-prollne II and poly-~.-serine, and for the proteins ~S-lactoglobulin, and pepsinogen, deviate from the straight line described by Equation 4. The two synthetic polymers, poly-L-proline II and poly~L-serine have been shown to have non-a-helical conformations (49,81,82,83) and recent evidence suggests that the two proteins may contain significant amounts of systematic, but non-a-helical conformations (84).…”

Section: Optical Rotatory Dispersion Of Proteins and Polypeptidesmentioning

confidence: 99%