New insight into the structure, reaction mechanism, and biological functions of neutral ceramidase.

Ceramidase (CDase) is an enzyme that hydrolyzes the N-acyl linkage between the sphingoid base and fatty acid of ceramide. These enzymes are classified into three distinct groups, acid (Asah1), neutral (Asah2), and alkaline (Asah3) CDases, based on their primary structure and optimum pH. Acid CDase catabolizes ceramide in lysosomes and is found only in vertebrates. In contrast, the distribution of neutral and alkaline CDases is broad, with both being found in species ranging from lower eukaryotes to mammals; however, only neutral CDase is found in prokaryotes, including some pathogenic bacteria. Neutral CDase is thought to have gained a specific domain (mucin box) in the N-terminal region after the vertebrate split, allowing the enzyme to be stably expressed at the plasma membrane as a type II membrane protein. The X-ray crystal structure of neutral CDase was recently solved, uncovering a unique structure and reaction mechanism for the enzyme. Neutral CDase contains a zinc ion in the active site that functions as a catalytic center, and the hydrolysis of the N-acyl linkage in ceramide proceeds through a mechanism that is similar to that described for zinc-dependent carboxypeptidase. This review describes the structure, reaction mechanism, and biological functions of neutral CDase in association with the molecular evolution, topology, and mechanical conformation. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.