Summary
Highlights
Proteins are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. They form vital structures in the body like hemoglobin, collagen, cell membrane components, enzymes, antibodies, and keratin. Amino acids are the monomers, dipeptides are dimers, and polypeptides are polymers of proteins.
Amino acids have a central carbon atom bonded to an amine group (NH2), a carboxylic acid group (COOH), a hydrogen atom, and an R-group. The R-group varies, with 20 different types. Glycine has the simplest R-group (a hydrogen), and cysteine contains sulfur, making it crucial for disulfide bonds. Amino acids can ionize in water, acting as both acids and bases. Identifying the amine and carboxylic acid groups helps locate the central carbon and R-group in complex structures.
A peptide bond forms between the amine group of one amino acid and the carboxylic acid group of another through a condensation reaction, releasing a water molecule. This C double bond O-NH linkage is the peptide bond. Multiple amino acids joined by peptide bonds form a polypeptide chain.
In a polypeptide, the presence of nitrogen indicates a protein. Peptide bonds (C double bond O-NH) help locate the central carbon atoms. The R-groups are the side chains attached to these central carbons. The video demonstrates how to identify common amino acids like glycine (R-group is H) and cysteine (R-group contains SH) within a polypeptide structure.
Beyond peptide bonds, four other types of bonds contribute to protein structure: hydrogen bonds (weak, but cumulatively strong, found between peptide groups or R-groups), disulfide bonds (strong covalent bonds between sulfur atoms of cysteine), ionic bonds (between charged R-groups or terminal ionized groups), and hydrophobic interactions (weak interactions between non-polar R-groups that cluster away from water). These bonds are ranked from weakest (hydrophobic) to strongest (covalent peptide and disulfide bonds).
Proteins fold into specific structures at four levels: primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids linked by peptide bonds. The secondary structure involves coiling (alpha-helix) or folding (beta-pleated sheet) due to hydrogen bonds between peptide groups. The tertiary structure is the complex 3D shape formed by coiling and folding of a single polypeptide chain, maintained by all four types of bonds. The quaternary structure involves the association of two or more polypeptide chains, with all bond types present.
Myoglobin is an example of a tertiary protein structure, composed of a single polypeptide chain that carries oxygen in muscle cells. Hemoglobin, conversely, is a quaternary protein, consisting of four polypeptide chains. While a protein may be classified by its highest level of structure, like hemoglobin being quaternary, it inherently possesses all lower levels of structure (primary, secondary, and tertiary).