Enantiomers living systems

Section 7 8 Both enantiomers of the same substance are identical m most of then-physical properties The most prominent differences are biological ones such as taste and odor m which the substance interacts with a chiral receptor site m a living system Enantiomers also have important conse quences m medicine m which the two enantiomeric forms of a drug can have much different effects on a patient... 

What are the facts of life One of the most striking is that all known living systems involve the same types of polymers, i.e., three varieties of homochiral biopolymers. That is, each variety is composed of unique molecular building blocks having the same three-dimensional handedness. Thus, with rare exceptions, the proteins found in cells are composed exclusively of the 1-enantiomers of 19 optically active amino acids (Fig. 11.1). Similarly, only D-ribose and 2-deoxy-D-ribose sugars are found in the nucleic acid polymers that make up the RNAs and DNAs, which are essential for protein synthesis in the cell and for the transmission of genetic information from one generation to the next. 

Enzymes in living systems are chiral, and they are capable of distinguishing between enantiomers. Usually, only one enantiomer of a pair fits properly into the chiral active site of an enzyme. For example, the levorotatory form of epinephrine is one of the principal hormones secreted by the adrenal medulla. When synthetic epinephrine is given to a patient, the (—) form has the same stimulating effect as the natural hormone. The ( + ) form lacks this effect and is mildly toxic. Figure 5-15 shows a simplified picture of how only the ( —) enantiomer fits into the enzyme s active site. 

Most of the molecules we find in nature are chiral—a complicated molecule is much more likely not to have a plane of symmetry than to have one. Nearly all of these chiral molecules in living systems are found not as racemic mixtures, but as single enantiomers. This fact has profound implications, for example, in the chemistry of drug design, and we will come back to it later. 

Lor example, because an acid reacts with a base to form a salt, a racemic mixture of a carboxylic acid reacts with a naturally occurring optically pure (a single enantiomer) base to form two diastereomeric salts. Morphine, strychnine, and brucine are examples of naturally occurring chiral bases commonly used for this purpose. The chiral base exists as a single enantiomer because when a chiral compound is synthesized in a living system, generally only one enantiomer is formed (Section 5.20). When an / -acid reacts with an 5 -base, an / ,5 -salt will be formed when an S -acid reacts with an... 

Why Nature uses only one enantiomer of most important biochemicals is an easier question to answer than how this asymmetry came about in the first place, or why L-amino acids and D-sugars were the favoured enantiomers, since, for example, proteins made out of racemic samples of amino acids would be complicated by the possibility of enormous numbers of diastereomers. Some have suggested that life arose on the surface of single chiral quartz crystals, which provided the asymmetric environment needed to make life s molecules enantiomerically pure. Or perhaps the asymmetry present in the spin of electrons released as gamma rays acted as a source of molecular asymmetry. Given that enantiomerically pure living systems should be simpler than racemic ones, maybe it was just chance that the L-amino acids and the D-sugars won out. 

How can this be We said in Chapter 14 that enantiomers are chemically identical, so how is it that we can distinguish them with our noses and bacteria can produce them selectively Well, the answer lies in a proviso to our assumption about the identity of enantiomers they are identical until they are placed in a chiral environment. This concept will underlie all we say in this chapter about how to make single enantiomers in the laboratory. We take our lead from nature all life is chiral, so all living systems are chiral environments. 

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