PROTEIN: A cellular macromolecule; a polymer of amino acids, linked together by peptide bonds, in a specific sequence.
Proteins are a cell’s building blocks, making up most of a cell’s dry weight, and perform nearly every activity that goes on inside cells. These proteins, are remarkably versatile in both their structure and their function. Protein is translated by a series of proteins from RNA, which is transcribed by another series of proteins from DNA. It’s a bit more complicated than just that, but then that is for another discussion. This concept of the linear progression from DNA to RNA to protein, is called the central dogma. RNA is made up of codons or triplets of nucleic acids, each of which code for a particular amino acid. There are 20 different amino acids, each with its own properties, which come together in a dictated sequence to form a polypeptide chain (poly=many, peptide=the covalent linkage between two amino acids). This polypeptide or amino acid chain is what is called the primary protein structure, the simplest form of the protein. Once this chain is made, the different side chains of the amino acids start to interact, for instance amino acids that are non-polar want to get away from the aqueous environment of the cell, and thus end up in the center of the structure, while amino acids that are polar like the environment rich in water, and so will end up on the outside of the structure, meanwhile amino acids with similar charges (both positive or negative) will repel, while opposite charges will attract. These various non-covalent interactions lead to the formation of secondary structures such as alpha-helixes and beta-pleated sheets. The interaction of these different secondary structures and the full three-dimensional organization of the polypeptide makes up the tertiary structure. Small proteins may be made up of a single polypeptide chain and so their tertiary structure will be their functional form. In larger proteins, however, such as hemoglobin, they require more than one subunit (or folded polypeptide chain), which will come together to form the quaternary structure by the same non-covalent forces that determined the initial protein folding.