Structure of human salivary α-amylase crystallized in a C-centered monoclinic space group

Human salivary α‐amylase (HSA) is a major secretory protein component of saliva and has important biological functions, including the initial digestion of starch. HSA acts as a monomer and mediates the hydrolysis of α‐1,4‐glucosidic linkages in oligosaccharides. To date, all published crystal structures of HSA have been crystallized as monomers in space group P212121. Here, the serendipitous purification, crystallization and ultimate structure determination of a HSA non‐crystallographic symmetry (NCS) dimer, while attempting to purify human carbonic anhydrase VI (HCA VI) from saliva using an affinity resin for α‐class carbonic anhydrases, is presented. On further investigation, it was shown that HSA could only be copurified using the affinity resin in the presence of HCA VI which is glycosylated and not the non‐glycosylated HCA II. The identification of the HSA crystals was carried out by peptide mapping and mass spectrometry. HSA was shown to have crystallized as an NCS dimer in space group C2, with unit‐cell parameters a = 150.9, b = 72.3, c = 91.3 Å, β = 102.8°. The NCS dimer crystal structure is reported to 3.0 Å resolution, with a refined Rcryst of 0.228. The structure is compared with the previously reported P212121 monomer structures and the crystal packing and dimer interface are discussed.