Introduction
There are 20 standard amino acids used by living organisms to construct biological molecules called proteins.
Proteins are essential molecules that perform a vast range of functions in living organisms. They serve as structural components (e.g., collagen in skin and keratin in hair), enzymes (e.g., DNA polymerase for DNA replication), hormones (e.g., insulin for blood sugar regulation), transporters (e.g., hemoglobin carrying oxygen in the blood), and immune defenders (e.g., antibodies fighting infections).
The 20 amino acids are linked by peptide bonds through dehydration synthesis, creating polypeptide chains that fold into complex three-dimensional structures essential for enzymatic activity, cell signaling, and structural support. The resulting proteins are each distinguished by its unique side chain (R-group), which determines its chemical properties—such as being hydrophobic, hydrophilic, acidic, or basic.
The 20 Standard Amino Acids and Their Abbreviations
Amino acids are designated by both three-letter and one-letter codes using the alphabet, primarily for convenience and efficiency in scientific communication and data handling.
Glycine (Gly, G)
Alanine (Ala, A)
Valine (Val, V)
Leucine (Leu, L)
Isoleucine (Ile, I)
Methionine (Met, M)
Phenylalanine (Phe, F)
Tryptophan (Trp, W)
Proline (Pro, P)
Serine (Ser, S)
Threonine (Thr, T)
Asparagine (Asn, N)
Glutamine (Gln, Q)
Tyrosine (Tyr, Y)
Cysteine (Cys, C)
Lysine (Lys, K)
Arginine (Arg, R)
Histidine (His, H)
Aspartic acid (Asp, D)
Glutamic acid (Glu, E)
The use case for these abbreviations are presented below:
Writing Protein Sequences: Scientists use both three-letter and one-letter codes to represent protein sequences in research papers, textbooks, and databases. For example, a short peptide sequence might be written as Ala-Gly-Val (three-letter) or AGV (one-letter).
Sequence Alignment: In bioinformatics, amino acid letter codes are essential for aligning protein sequences to identify similarities and differences. This is crucial for understanding protein evolution, function, and structure.
Database Entries: Protein databases like UniProt and NCBI use amino acid letter codes to store and display protein sequence information.
Genetic Code Tables: The genetic code, which translates DNA into protein sequences, is often represented using three-letter codes for amino acids.
Visual Representation: One-letter codes are particularly useful for visually representing long protein sequences, such as in figures or diagrams.
Abbreviations are used to denote amino acid substitutions resulting from genetic mutations. This shorthand is crucial for concisely describing changes in protein sequences. The most common way to represent amino acid substitutions is: Original Amino Acid - Position - New Amino Acid. E.g. Val600Glu: This indicates that the amino acid at position 600 in the protein has changed from Valine (V) to Glutamic acid (E).
Amino Acid Classifications Based on Chemical Properties
The chemical properties influence the final 3D structure of the resultant proteins, and impacts:
Biological function
Mediates interactions with other molecules
Regulates its activity
Influence its localization within the cell
Nonpolar (Hydrophobic) Amino Acids
Glycine (Gly, G)
Alanine (Ala, A)
Valine (Val, V)
Leucine (Leu, L)
Isoleucine (Ile, I)
Methionine (Met, M)
Phenylalanine (Phe, F)
Tryptophan (Trp, W)
Proline (Pro, P)
Polar (Hydrophilic) Amino Acids
Serine (Ser, S)
Threonine (Thr, T)
Asparagine (Asn, N)
Glutamine (Gln, Q)
Tyrosine (Tyr, Y)
Cysteine (Cys, C)
Charged Amino Acids
Positively Charged (Basic):
Lysine (Lys, K)
Arginine (Arg, R)
Histidine (His, H)
Negatively Charged (Acidic):
Aspartic acid (Asp, D)
Glutamic acid (Glu, E)
Additional Amino Acids
Selenocysteine (Sec, U): considered the 21st amino acid, incorporated into some proteins through a special mechanism.
Pyrrolysine (Pyl, O): found in certain archaea and bacteria, considered the 22nd amino acid in some contexts.
Conclusion
The 20 amino acids are the foundation of all proteins, shaping their structure and function in living organisms. Each amino acid contributes unique chemical properties that influence protein stability, interactions, and biological roles. Whether in enzymes, structural proteins, or signaling molecules, amino acids are essential to life itself.
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