Amino acids are fundamental organic molecules that serve as the building blocks of proteins, essential for virtually all biological processes. These molecules exhibit “handedness,” a property called chirality, which means they exist in two mirror-image forms. Understanding which form dominates and why offers insights into life’s molecular architecture.
What is Handedness in Amino Acids?
Chirality, or handedness, describes a molecule that cannot be superimposed on its mirror image, much like a person’s left and right hands. All amino acids, except glycine, possess a central carbon atom, called the alpha-carbon, bonded to four different groups. This unique arrangement creates two distinct spatial configurations, known as enantiomers.
Amino acids are classified as “D” (dextro) or “L” (levo) based on the orientation of their amino group around the alpha-carbon when drawn in a specific two-dimensional representation called a Fischer projection. An L-amino acid has the amino group on the left, while a D-amino acid has it on the right. These designations refer to structural configuration and do not directly indicate how the molecules rotate polarized light.
The Dominance of L-Amino Acids
In the vast majority of biological systems on Earth, particularly within the proteins that make up all known life forms, L-amino acids are overwhelmingly predominant. Out of the 20 common amino acids used to build proteins, 19 are chiral and exist almost exclusively in their L-form. This strong bias towards L-amino acids is often referred to as biological homochirality.
Why L-Amino Acids Rule
The reason for the exclusive use of L-amino acids in proteins is not fully understood, but several hypotheses suggest functional advantages. One significant factor is the efficiency of protein folding and stability. Homochiral chains of amino acids can fold into precise, stable three-dimensional structures more effectively than mixtures of D and L forms, which would lead to less stable and functional proteins.
Enzymes are stereospecific; they are highly selective and recognize and interact with only one chiral form of a molecule. Since the enzymes responsible for synthesizing proteins are composed of L-amino acids, they preferentially produce and incorporate L-amino acids into new protein chains. This creates a self-reinforcing system. The concept of a “homochiral world” suggests that this handedness may have emerged very early in life’s origins, potentially from an initial small imbalance that was amplified over time.
The Surprising World of D-Amino Acids
Despite the overwhelming dominance of L-amino acids in proteins, D-amino acids are not entirely absent from nature and play specific, crucial roles. They are found in various biological contexts. D-amino acids are abundant components of the peptidoglycan cell walls of bacteria, providing structural integrity and resistance to enzymatic degradation by host proteases. D-alanine and D-glutamate are common in bacterial cell walls, helping bacteria withstand environmental stresses and immune attacks.
D-amino acids are also found in certain antibiotics, such as gramicidin, tyrocidine, and valinomycin, contributing to their stability and ability to disrupt bacterial cell walls. In higher organisms, D-amino acids like D-serine and D-aspartate function as signaling molecules. D-serine acts as a neurotransmitter in the brain, involved in learning and memory, while D-aspartate is associated with neurogenesis and endocrine systems. D-amino acids, particularly D-aspartic acid, can also appear in proteins of higher organisms due to the spontaneous racemization of L-amino acids over time, especially in aged tissues like the eye lens, and may be linked to age-related conditions.