a protein letter – but not always… When we talk about amino acids, we’re usually referring to one of the 20 (common) proteinogenic amino acids – that is to say one of the amino acids that gets incorporated into a protein during the protein-making process called translation. There are also some other amino acids that don’t go into proteins (like creatine) and even the proteinogenic ones aren’t always found in proteins – they can have other roles such as acting as chemical messengers or building blocks for other molecules. What characterizes an amino acid is that it has an amine group (-NH₂ or -NH₃⁺ depending on pH) and a carboxylic acid group (-(C=O)-OH (or carboxylate (-(C=O)-O⁻) depending on pH). In addition to this “generic backbone” part, amino acids have unique side chains (aka R groups) that give them different properties. Some side chains are big & bulky, others are small and flexible. Some are (usually) charged, others are neutral, etc. So when you link amino acids together through peptide bonds to form long polypeptide chains that fold up into functional proteins, those side chains, which stick off from the conjoined backbones kinda like charms on a charm bracelet, influence how the proteins fold and function.
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