Proteins are macromolecules that play important roles in life processes such as catalyzing biological reactions, forming DNA strands, sending signals within systems, and transportation of substances in the body. They are composed of amino acid subunits, which in specific combinations form peptides linked by a C-N peptide bond. Chains of peptides are called polypeptides, or proteins.
Proteins are folded into specific spatial conformations by non-covalent bonds such as hydrogen bonding, ionic bonding, disulfide bonds, Van der Waals forces and hydrophobic interactions. There are four types of structures: primary, secondary, tertiary and quaternary. Primary structure is the linear amino acid sequence, whilst the secondary structure is coiling or folding of the amino acid sequence. The tertiary structure is the three-dimensional globular structure of the protein, and the quaternary structure is the interaction of multiple polypeptides. All these structures denote the final shape of a specific protein.
The synthesis of proteins is done by a two-step process of transcription and translation. Transcription is the process of forming a RNA strand in the cytoplasm, which is similar to the DNA strand by creating a complementary mRNA structure (messenger RNA) in the nucleus. The difference between RNA and DNA is the presence of uracil in RNA, which replaces thymine in the DNA strand. Translation occurs in the cytoplasm where ribosomes use tRNA (transfer RNA) to decode the RNA strand, by selecting appropriate amino acids to form a peptide chain. There are four stages in translation: activation, initiation, elongation and termination.
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