4.12: Chirality in Nature
Chirality is the most intriguing yet essential facet of nature, governing life’s biochemical processes and precision. It can be observed from a snail shell pattern in a macroscopic world to an amino acid, the minutest building block of life. Most of the snails around the world have right-coiled shells because of the intrinsic chirality in their genes. All the amino acids present in the human body exist in an enantiomerically pure state, except for glycine - the sole achiral amino acid. The chirality of amino acids has a significant consequence on the symmetry and function of naturally occurring proteins and enzymes. With 268 chiral centers, human chymotrypsin could exist in 2268 possible configurations if each amino acid took either of the enantiomeric forms. However, the role of chirality has ordained a single chiral chymotrypsin as the selective digestive enzyme.
Another critical aspect in the cascade of biochemical processes is that most enzymes interact with only one of the enantiomers due to their chirality. Consequently, enantioselectivity arises, like a lock-and-key mechanism, where only one enantiomer can fit into the enzyme’s binding site. This has a significant implication in the domain of drug design, where each enantiomer can induce a different effect. The role of chirality was brought to light in a devastating way nearly five decades ago when the drug thalidomide was prescribed for the treatment of morning sickness in pregnant women. Ever since, the properties of each enantiomer have been ascertained for every drug designed.
Most interestingly, this facet of chirality extends from the microcosm to the macrocosm. When Pasteur discovered the connection between optical activity and molecular chirality, it led him to conjecture that even the forces of nature are chiral. This has now been proven across the universe in the weak interactions between fundamental particles, which can violate parity symmetry.