Question

What are amino acids? What do we mean by their handedness? How do amino acids offer further evidence for a common ancestor for all life on Earth?

Short answer

Amino acids are protein-building molecules with handedness or chirality, indicating a common ancestor using L-amino acids in all life forms.

Step-by-step solution

Definition of Amino Acids

Amino acids are organic molecules that serve as the building blocks of proteins. Each amino acid has a basic structure that includes a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain known as the R group, which determines the characteristics and identity of each amino acid.

Understanding Handedness of Amino Acids

The handedness, or chirality, of amino acids refers to the spatial arrangement of their atoms. Most amino acids are chiral, meaning they exist in two mirror-image forms called enantiomers: left-handed (L-form) and right-handed (D-form). In biological systems, the L-form of amino acids predominates, meaning living organisms primarily use one-handedness in their molecular structures.

Evidence of Common Ancestry from Amino Acids

The fact that nearly all life on Earth exclusively uses L-amino acids to build proteins provides evidence for a common ancestor. This uniformity suggests that early life forms adopted L-amino acids due to chance, and subsequent life forms inherited this trait, pointing to a shared ancestry.

Key concepts

Protein Structure

Proteins are essential molecules found in every cell of our bodies and are crucial for countless biological processes. They are made up of smaller units called amino acids, which link together in specific sequences to form proteins. This sequence determines the protein's final shape and function. The protein structure can be divided into four levels:

• Primary structure: This is the unique sequence of amino acids in a protein chain. The order of amino acids is determined by the genetic code.
• Secondary structure: The amino acid chain may twist into a helix or fold into a pleated sheet. These shapes are stabilized by hydrogen bonds.
• Tertiary structure: The overall 3D shape of a single protein molecule. This structure is stabilized by interactions between the side chains of the amino acids.
• Quaternary structure: Some proteins consist of more than one amino acid chain. The quaternary structure refers to the arrangement of these chains into a functional protein.

Proteins' diverse structures allow them to perform a wide range of biological functions, including acting as enzymes, structural components, and transport molecules. Understanding protein structure is essential for fields like biochemistry and medicine, as it provides insights into how proteins work and how their dysfunction can lead to diseases.

Chirality

Chirality is a property of asymmetry important in several branches of science. In chemistry, chirality refers to molecules that are mirror images of each other, similar to how our left and right hands are mirrors but not identical. These molecules are known as enantiomers.

In amino acids, chirality is manifested in the presence of L- and D-forms. The central carbon atom, called the alpha-carbon, is bonded to four different groups: an amino group, a carboxyl group, a hydrogen atom, and an R group (side chain). This configuration makes all amino acids (except glycine) chiral.

Chirality is crucial because biological systems are selective and typically only use one enantiomer. For amino acids, the L-form is almost exclusively used in natural proteins. This selectivity influences how proteins are synthesized and function, as the shape and fit of molecules in biological reactions are pivotal.

Chirality is not just limited to amino acids but is a critical factor in the effectiveness of pharmaceuticals, the behavior of materials, and even the study of the origins of life.

Common Ancestry

The concept of common ancestry is fundamental in understanding the evolution of life on Earth. It suggests that all living organisms share a common origin. One of the compelling pieces of evidence supporting this idea is the universal use of L-amino acids by all known life forms.

Since amino acids can exist as either L- or D-forms, the exclusive use of one form (L-amino acids) in the proteins of all living organisms suggests a shared genetic heritage. This uniformity implies that early life on Earth likely started with L-amino acids, and this characteristic was conserved through evolutionary history.

Further evidence of common ancestry includes:

• The similarity of genetic codes among diverse organisms.
• Conserved biological pathways and processes.
• Structures and functions shared by species across the tree of life.

This shared use of L-amino acids acts as a molecular fingerprint, linking all organisms to a universal ancestor, underscoring the interconnectedness of life across billions of years.