Peptide bonds and amino acids

Proteins are made from amino acids. All amino acids have a central carbon atom bonded to an amino group (-NH2) and a carboxylic acid group (-COOH).
Amino acids are joined together by peptide bonds, which are covalent bonds.
A peptide bond forms when one amino acid loses a hydroxyl (-OH) group from its carboxyl group and another loses a hydrogen atom from its amino group, resulting in the release of a water molecule.
Chains of amino acids linked by peptide bonds are called polypeptides, which form proteins.
Protein structure is split into four levels - primary, secondary, tertiary and quaternary.

Primary structure

The primary structure refers to the sequence of amino acids in a polypeptide chain.
The primary structure is unique to each protein.
A change in a single amino acid can completely alter the properties of the protein.
The primary structure is usually written out following the format amino_acid_1 - amino_acid_2 - amino_acid_3 - etc.
For example, the polypeptide shown would be written as Asn-Gly-Phe-Glu-...

The secondary structure describes the coiling or folding of the polypeptide chain.
Common secondary structures include the α-helix and β-pleated sheet, which are maintained by hydrogen bonds between amino acids.
In diagrams, α-helices are represented as coils or cylinders and β-pleated sheets as arrows.
The hydrogen bonds are formed between the H atoms on NH or CH and the oxygen in C=O. Note that atoms in the R-group are not involved.

The tertiary structure is the overall three-dimensional structure of a single polypeptide, resulting from further coiling and folding of the secondary structures.
It is stabilized by various bonds between R groups, including hydrogen bonds, disulfide bonds, ionic bonds, and hydrophobic interactions.
Hydrophobic R groups tend to orient themselves inwards to avoid water, influencing the protein's overall shape.
Hydrophilic R groups orient themselves outwards to form hydrogen bonds with water.

Quaternary structure arises when a protein consists of two or more polypeptide chains.
The polypeptide chains are held together by the same types of bonds that stabilize tertiary structure.
Hemoglobin, which has four polypeptide chains, is an example of a protein with quaternary structure.
Most enzymes and proteins utilised by the human body are quaternary in structure.