When amino acids are linked by hydrogen bonds, they form the secondary structure of a protein. Specifically, the hydrogen bonds form between the carbonyl group of one amino acid and the amino group of another amino acid. This results in the formation of two types of secondary structures: alpha helices and beta sheets.
In an alpha helix, the polypeptide chain is coiled into a spiral shape, held together by hydrogen bonds between the carbonyl group of one amino acid and the amino group of another amino acid located four positions down the chain.
In a beta sheet, the polypeptide chain is folded back and forth, forming a flat, sheet-like structure. The hydrogen bonds form between adjacent strands of the sheet, holding them together.
The secondary structure of a protein is important because it helps to determine the overall shape of the protein, which in turn determines its function. The hydrogen bonds that hold the secondary structure in place are weaker than the covalent bonds that hold the primary structure (the sequence of amino acids) in place, which allows the protein to be flexible and to change shape as needed to carry out its function.