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The properties of proteins are determined not only by the sequence
of amino-acid residues in the polypeptide chains, but also by the
con- figuration of the chains-the way in which the chains are
coiled or folded. It is probable that denaturation, the loss of
some of the specific properties of a native protein, may in many
cases be the result simply of a change in configuration of the
polypeptide chains, without any change whatever in the sequence of
amino-acid residues. During the past few years great progress has
been made in the attack on the determination of the sequence of
amino-acid residues in the poly- peptide chains of proteins,
through the work of SANGER and his collabora- tors (Io9, IIO) and
of other investigators. There has also been significant progress in
the attack on the problem of the configuration of polypeptide
chains, largely through the application of the X-ray diffraction
technique.
Sulfonium compounds have been known to the organic chemist for
almost 90 years. During most of this period they remained
curiosities, well suited for didactic purposes to illustrate
certain similarities to ether adducts and to organic ammonium
compounds. Their exploration remained largely in the academic
realm, because no unusual practical applications were found in
spite of a steady increase in the attention paid to them. An event
that occasionally puts a class of compounds into the center of
interest is the recognition of its occurrence in nature and of
special biological significance. The first natural sulfonium
compound was discovered less than 20 years ago, somewhat by chance
rather than as the climax of some coveted biochemical problem.
Gradually, however, the unique role of sulfonium compounds,
especially of S-adenosyl- methionine, in enzymatic group transfer
reactions such as trans- methylation was recognized, and the recent
upsurge of interest has been exceptional. The present review is
directed toward an interpretation of the properties and functions
of biological sulfonium compounds with frequent reference to their
structural chemistry. The data presented here may suggest novel or
improved analytical techniques and new interpretations of the
mechanism of group transfer. Furthermore, a stimulus may be derived
for the design of sulfonium compound analogues. The availability of
competitive analogues of biochemical key compounds has aided in the
understanding of the function of virtually all metabolic
intermediates, prosthetic groups of enzymes, vitamins, amino acids,
and other compounds. The pattern will be similar in sulfonium
biochemistry.
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