Amino Acids and the Peptide Bond

Every amino acid has the same backbone: a central carbon bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen, and one variable R group. That shared core is what lets them all link up the same way, while the R group is what makes each of the 20 amino acids chemically distinct. R groups sort into three main families: nonpolar (hydrophobic, like the plain hydrocarbon chain in leucine), polar (uncharged but able to form hydrogen bonds, like the -OH in serine), and charged (acidic or basic, like the -COO- in aspartate or the -NH₃+ in lysine).

Amino acids join through a peptide bond: the carboxyl group of one amino acid reacts with the amine group of the next, releasing a water molecule in a dehydration synthesis reaction. This is the same water-releasing pattern used to build the other carbon polymers in the unit, carbohydrates and nucleic acids, just with a different linkage chemistry. Chain n amino acids together and you form exactly n minus 1 peptide bonds and release n minus 1 water molecules.

The order of amino acids, called the primary sequence, is what really matters, not just how many you have. Swap the sequence and you change where polar, nonpolar, and charged R groups end up along the chain, which changes how the chain folds in water. A hydrophobic R group that lands among polar ones tends to get buried away from water as the chain folds, a first hint of the secondary and tertiary structure built from this same primary sequence later in the unit.