Examples of hydrogen bond interactions from conserved, buried residues to mainchain atoms in polyproline helices. Representative structures were chosen for each family based on resolution; residues are coloured by atom type with buried, conserved polar residues shown in magenta. Hydrogen bonds are shown in black. Two examples of arginines forming hydrogen bonds to polyproline helices in A) the α/β hydrolases [PDB: 2bce] and B) the alcohol dehydrogenases - polyproline interaction on the right [PDB: 2ohx]. C) Two tyrosines in the eukaryotic serine proteinases [PDB: 1avw] forming hydrogen bonds to polyproline helices which form an interaction site with trypsin.