Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
The Composition of Living Matter: Biomolecules
Many biomolecules are asymmetrical

The tetrahedral arrangement of single bonds formed by a carbon atom imparts another remarkable property to certain Organic compounds, which is of paramount importance in biology. Whenever a carbon atom in an organic molecule is bonded to four different atoms or functional groups, this atom is said to be asymmetric because it can exist in two isomeric forms known as enantiomers, which differ in their spatial configuration. As shown in Fig. 3-8, enantiomers are related to each other as an object and its non-superimposable mirror image. Enantiomers, also called optical isomers or stereoisomers, behave identically in Chemical Reactions but differ in a very characteristic physical property, namely, The ability to rotate the plane of polarization of plane-polarized light. If a solution containing one type of enantiomer rotates the plane of polarization to the right, a solution of the other enantiomer will rotate it to the left; the angle of rotation can be measured using a polarimeter. Compounds whose molecules lack asymmetric carbon atoms are incapable of rotating the plane of polarization of plane-polarized light.

The amino acid Alanine, depicted in Fig. 3-8, is an asymmetric molecule because its central carbon atom has four different substituents: a methyl group, an amino group, a carboxyl group, and a hydrogen atom. As we shall see later, the two enantiomers of alanine are structurally non-superimposable mirror images of each other. These two forms of alanine are related to one another in the same way as a right hand is to a left hand, and we know well from experience that a left-hand glove cannot be worn on the right hand. Because compounds containing asymmetric carbon atoms occur in two forms that can be regarded as left-handed and right-handed, they are termed chiral compounds (from the Greek word "cheiros", meaning hand). Accordingly, the central, or asymmetric, atom of chiral molecules is referred to as a chiral atom or chiral center.

Besides Amino Acids, many other organic Biomolecules also possess chiral properties and contain one or more asymmetric carbon atoms. An example of such compounds is the ubiquitous sugar glucose, whose molecule contains no fewer than five asymmetric carbon atoms. In living organisms, chiral molecules usually occur in only one of the two possible forms. For instance, amino acids, and alanine in particular, are found in Proteins in only a single chiral form. Similarly, glucose, the primary structural unit of starch, is discovered in biological systems in only one of its numerous chiral forms. Conversely, when a chemist synthesizes an organic molecule with a single asymmetric carbon atom in the laboratory using conventional non-biological reactions, both possible chiral forms are produced with equal probability, resulting in an equimolar mixture of the two enantiomers. These two chiral forms can be separated from such a mixture only by means of special physical techniques. In living Cells, chiral biomolecules are synthesized through enzyme-mediated processes in such a way that only one of the two possible enantiomers is generated. This occurs because the enzyme molecules themselves possess a chiral Structure.

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Fig. 3-8. Chiral molecules. A. If a carbon atom is bonded to four different groups or atoms (A, B, X, Y), the latter can be arranged in two ways, forming two structures that are non-superimposable mirror images of each other. Such a carbon atom is asymmetric and is called a chiral atom or chiral center. B. When a carbon atom is bonded to only three different groups or atoms, only one spatial configuration is possible; such a molecule is symmetric or achiral. In this case, the molecule can also be represented as two mirror images that are, however, superimposable upon each other. If the molecule on the left in the figure is rotated counterclockwise (with the axis of rotation passing vertically through atom A), it will coincide with the molecule on the right. C. Alanine is a chiral molecule because its central carbon atom is bonded to four different substituents, which allows for the existence of two structures that are non-superimposable mirror images of each other. The two different chiral forms of alanine are designated as D-alanine and L-alanine. The physical meaning of the letters D and L used to denote the chiral forms of asymmetric molecules is discussed in Chapter 5.

The stereospecificity inherent in many biomolecules is a hallmark of the molecular logic of living cells, further compellingly demonstrating that the three-dimensional structure of biomolecules is crucial for their biological Functions. We will examine chiral molecules and The phenomenon of stereoisomerism in greater detail when we study amino acids (Chapter 5) and sugars (Chapter 11).