Question

Amino acids, such as alanine shown here, are the monomer building blocks of proteins (Chaprer 13). Most amino acids have a handedness that is, for each of these amino acids there is a possible mirror image. Interestingly, amino acids that occur in proteins are always of the left-handed sort. Why are right-handed amino acids never found in proteins?

Short answer

Proteins evolved to use only L-amino acids due to enzyme specificity and structural necessity.

Step-by-step solution

Understanding Amino Acid Handedness

Amino acids have a property called chirality, meaning they exist in two mirror image forms, called enantiomers: left-handed (L) and right-handed (D). L-amino acids and D-amino acids are non-superimposable mirror images of each other. Proteins are composed of chains of amino acids and are stereospecific, utilizing only L-forms.

Biological Evolution of L-Amino Acids

During the evolution of life, L-amino acids became the building blocks in the proteins of living organisms. This specificity is likely due to a mixture of factors including, specific enzyme recognition and the chemical environment in early Earth's biology, leading to the dominance of L-amino acids in living systems.

Protein Function and Structure

Proteins have specific functions and structures, and they rely on the precise arrangement of amino acids in their chain. Using D-amino acids in proteins could disrupt the normal folding patterns and interaction required for biological activity, as the three-dimensional configuration is crucial for enzymatic function.

Key concepts

L-amino acids

Amino acids are the building blocks of proteins and they have a property known as chirality. This means they exist in mirror image forms, similar to how your left and right hands are mirror images of each other. These mirror images are called enantiomers, labeled as L (left) and D (right). However, in the proteins found within living organisms, you will observe only L-amino acids being used. This is a crucial detail because it plays a significant role in the way proteins function in biological systems.

Why L and not D? Well, this is due to the enzymes that are involved in protein synthesis. These enzymes are highly specific and can only recognize and bind to L-enantiomers. This specificity ensures that proteins fold correctly, allowing them to perform their intended biological functions. If there were a mix of L and D forms, it could potentially disrupt the protein's stability and functionality.

Protein structure

The structure of proteins is both intricate and extraordinary, being vital to their function in biological systems. Proteins are composed of long chains of amino acids that fold into specific three-dimensional shapes, which determine their action in biological processes. The exact order and arrangement, known as the protein's primary structure, drive the way the protein folds into its three-dimensional form, influencing its secondary, tertiary, and quaternary structures.

• Secondary structure: This includes elements like alpha helices and beta sheets, formed by hydrogen bonding along the backbone of the amino acid chain.
• Tertiary structure: Refers to the entire three-dimensional shape of a single protein molecule.
• Quaternary structure: The assembly of multiple protein molecules, or protein subunits, into a functioning complex.

Using only L-amino acids is essential in maintaining the delicate folding process, as even a small alteration, like incorporating D-amino acids, could lead to misfolding. Misfolded proteins could lose their function or, worse, aggregate, causing diseases such as Alzheimer's.

Biological evolution

Biological evolution has been pivotal in shaping the use of L-amino acids in proteins. When life first began on Earth, various forms of amino acids were likely present, including both L- and D-enantiomers. However, over billions of years, a preference for L-amino acids became predominant in biological systems.

Several factors might have contributed to this evolutionary development:

• Chemical Environment of Early Earth: Conditions favored the formation or stability of L-amino acids.
• Stereochemical Fitness: L-amino acids might have provided proteins with properties that gave them an advantage in efficiency or stability, aiding cellular processes.
• Enzymatic Preference: Early catalytic mechanisms may have inherently favored L-amino acids, thus rooting their use in biological systems.

This evolutionary bias became solidified, as it provided a consistent framework for life to build upon. Thus, in a sense, life as we understand it today was shaped, in part, by this foundational choice of L-amino acids.