Examples of hydrogen bond interactions from conserved, buried residues to mainchain atoms in centre strands. 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. A) A serine residue within a coil forming hydrogen bonds to two strands that have deviated away from each other in the haloperoxidases [PDB: 1b6g]. Examples of polar residues that form hydrogen bonds to an adjacent strand that extends further than its neighbour, including serines in B) the pancreatic ribonuclease family [PDB: 7rsa] and C) the cyclodextrin glycosyltransferases [PDB: 1qhp], D) a threonine in the aldehyde oxide and xanthine dehydrogenases (domains 1&2) [PDB: 1fo4] and E) a cysteine in the papain family cysteine proteinase [PDB: 1mem]. F) Two threonines that form hydrogen bonds to each other's mainchain amide atoms as well as atoms within strands (one central, one edge) in the aspartic proteinases [PDB: 3app].