Why are there 20 different amino acids in polypeptides synthesized on ribosomes?

SouravNovember 8, 2024

Answer

The presence of 20 different amino acids in polypeptides synthesized on ribosomes is fundamental to the diversity and functionality of proteins in biological systems. Here’s an overview of why these 20 amino acids are essential:

1. Genetic Code and Amino Acid Specification

Codon-Amino Acid Relationship

• The genetic code consists of sequences of three nucleotides (codons) in messenger RNA (mRNA) that specify which amino acids are to be incorporated into a growing polypeptide chain. Since there are 64 possible codons (4 nucleotides raised to the power of 3), but only 20 amino acids, this redundancy allows for some codons to specify the same amino acid, providing a buffer against mutations.

Unique Codons for Each Amino Acid

• Each of the 20 amino acids is encoded by one or more specific codons. For example, methionine is encoded by the AUG codon, which also serves as the start codon for protein synthesis. This specificity ensures that proteins can be accurately synthesized according to genetic instructions.

2. Diversity of Protein Functions

Structural and Functional Roles

• The variety of amino acids allows for an extensive range of protein structures and functions. Each amino acid has unique properties (e.g., size, charge, polarity) due to its side chain (R group), which influences protein folding and stability.
• This diversity enables proteins to perform a multitude of roles in the body, including:
  • Enzymatic activity: Catalyzing biochemical reactions.
  • Structural support: Forming cellular structures (e.g., collagen).
  • Transport: Carrying molecules across membranes (e.g., hemoglobin).
  • Signaling: Acting as hormones or receptors.

3. Essential vs. Nonessential Amino Acids

Essential Amino Acids

• Among the 20 amino acids, nine are classified as essential, meaning they cannot be synthesized by the human body and must be obtained from dietary sources. These include:
  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine.

Nonessential Amino Acids

• The remaining amino acids can be synthesized by the body from other compounds, making them nonessential. This classification emphasizes the importance of dietary intake for certain amino acids to support protein synthesis and overall health.

4. Role of tRNA and Ribosomes

tRNA Specificity

• Each amino acid is linked to a specific transfer RNA (tRNA) molecule by an enzyme called aminoacyl-tRNA synthetase. This enzyme ensures that the correct amino acid is attached to its corresponding tRNA, which then delivers it to the ribosome during protein synthesis.
• The ribosome reads the mRNA sequence and facilitates the formation of peptide bonds between adjacent amino acids, allowing for polypeptide elongation until a stop codon is reached