Proteins
Proteins are complex macromolecules composed of long chains of amino acids, which contain carbon, hydrogen, oxygen and nitrogen. The formation of proteins begins with amino acids linking together via peptide bonds that form between the carboxyl group of one amino acid and the amino group of another. This bond is established through a condensation reaction, where a molecule of water is released as the bond is formed.
There are 20 different amino acids that serve as the building blocks of proteins, each with a unique side chain (R-group) that influences the protein’s properties and function. These amino acids are classified into essential (obtained from the diet) and non-essential (synthesised by the body). The sequence and arrangement of these amino acids, linked by peptide bonds, determine a protein’s specific structure and role in the organism.
Primary structure: The primary structure of a protein is its linear sequence of amino acids, linked by peptide bonds. This sequence is determined by the genetic code and dictates the protein’s overall structure and function. The specific order of amino acids is crucial because even a single change in this sequence can lead to altered protein function or disease. For instance, a single amino acid substitution in haemoglobin can result in sickle cell anaemia.
Secondary structure: The secondary structure refers to the local folding patterns within a polypeptide chain, stabilised by hydrogen bonds. The two main types of secondary structures are alpha-helices and beta-pleated sheets. Alpha-helices are coiled structures where hydrogen bonds form between every fourth amino acid, stabilising the helix. Beta-pleated sheets consist of parallel or antiparallel strands linked together by hydrogen bonds, forming a sheet-like structure.
Tertiary structure: The tertiary structure is the overall three-dimensional shape of a single polypeptide chain, formed by the further folding of the secondary structures. This level of structure is stabilised by a variety of interactions, including hydrogen bonds, ionic bonds, disulphide bridges and hydrophobic interactions. The tertiary structure is crucial for the protein’s specific function, as it determines the spatial arrangement of functional groups and active sites.
Quaternary structure: The quaternary structure refers to the assembly of multiple polypeptide chains, or subunits, into a functional protein complex. These subunits can be identical or different, and their interactions are stabilised by the same types of forces that maintain tertiary structure. Haemoglobin, for example, has a quaternary structure consisting of four subunits (two alpha and two beta chains), each capable of binding oxygen, which allows it to transport oxygen efficiently in the blood.
