Plant lysozymes

Abstract

Structural and functional features of plant lysozymes are reviewed. All lysozymes also have chitinase activity, but not all plant chitinases are also lysozymes. However, for many chitinases it is not yet known if they also possess lysozyme activity. Enzymes with lysozyme activity occur in different, structurally unrelated, families of chitinases. Plant chitinases with lysozyme activity are basic enzymes with high isoionic points. Their lysozyme activities have a shart pH optimum around pH 4.5-5.0, while they show chitinase activities in a much broader pH range. High lysozyme activities are observed at low ionic strength values (0.05). The X-ray structure of a lysozyme/chitinase from latex of the rubber tree, Hevea brasiliensis, is presented. This enzyme is also known under the name hevamine. It belongs to the family 18 or h-type chitinases (also called class III chitinases). The structure consists of an alpha/beta barrel fold, which has not been found in other chitinase or lysozyme structures. A glutamic acid residue may be catalytically active in the substrate-binding cleft of the enzyme. Other plant lysozymes are homologous with the family 19 or b-type chitinases (class I, II and IV). The X-ray structure of barley chitinase, a representative of this family with negligible lysozyme activity, has a similar folding as found in animal and phage lysozymes.