Amino Acids and Protein Structures: A Comprehensive Look

Kinds of Amino Acids

Amino acids can be classified into several categories:

• Nonpolar:
  • Aliphatic
  • Aromatic
• Polar Uncharged
• Charged:
  • Acidic (Anionic)
  • Basic (Cationic)

Proteins: Building Blocks of Life

Proteins are biopolymers formed by molecular chains of 20 different amino acids, each with its own characteristics.

Protein Structures

Primary Structure

The primary structure is characterized by the specific composition of amino acids and their sequential order within the polypeptide chain.

Secondary Structure

The secondary structure refers to the folding of the primary structure due to the formation of hydrogen bonds between the -C=O group of one peptide bond and the -NH group of another. This results in the side chains (R groups) adopting specific spatial arrangements along the peptide chain.

Tertiary Structure

The tertiary structure is the overall three-dimensional conformation adopted by different regions of the polypeptide chain. It arises from interactions between the R side chains along the chain. There are two main types:

• Fibrous Proteins: These have a simple structure.
• Globular Proteins: These have a more complex tertiary structure resulting from the successive folding of the secondary structure, forming a spherical, compact, and usually water-soluble protein. In globular proteins, the different structural levels are arranged hierarchically.

Structural Domains of Proteins

Protein domains are formed by specific combinations of alpha-helices and beta-sheets, folded into stable and independent structures. These domains are particularly compact and stable, performing specific functions. The same domains can often be found in different proteins.

Quaternary Structure

The quaternary structure is only present in proteins composed of multiple peptide chains, either identical or different. The association between these chains, each with its own tertiary structure, is stabilized by weak bonds.

• Fibrous Proteins: Their quaternary structure is based on the association between different peptide units, resulting in large supramolecular complexes.
• Globular Proteins: Their quaternary structure is formed by the association of two or more polypeptide chains, each with its own tertiary structure.

Quaternary Structure and Biological Function

The quaternary structure is responsible for the protein's biological activity. Out of all possible amino acid sequences, only those that adopt stable conformations and can perform a wide range of functions are present in proteins.