High-resolution electron microscopic studies on the quaternary structure of ornithine aminotransferase from pig...

The ornithine aminotransferase (OAT) from pig kidney has been studied on the basis of high-resolution electron microscopy and the morphological appearance of the apoenzyme and holoenzyme have been examined. The quaternary structure of the OAT molecules in the presence of 5 mM pyridoxal 5'-phosphate could be established. The enzyme molecule appears to be built up of two morphological units, called M1. The native holoenzyme, termed morphological unit M2, measures 10.9 nm in length and 5.8 nm in width and its molecular mass is approximately 168 kDa, based on electron microscopical calculations. Since the enzyme is composed of only one type of 45-kDa subunit, the holoenzyme is a homotetramer. Each M1 is composed of two subunits and, as seen in top-view projection, has an oval to triangular shape. Upon tilting to 40 degrees the triangular shape changes into three distinct centers of mass. This morphological differentiation reflects the inner organization of M1, i.e. the shape of the individual subunit deviates from strictly globular proteins. This observation is compatible with the notion that the 45-kDa subunit consists of one large and one small domain. By tilting to 40 degrees, both large domains in M1 represent two of the three centers of mass, while the third center of mass is attributed to the superposition of both small domains. Thus, the four domains of both subunits in M1, in accordance with the triangular top-view projection, are quasi-tetrahedrally arranged. Since the change in shape of M1 upon tilting is only obvious in one of the two halves of the native OAT, it suggests that both morphological units of M2 are oriented asymmetrically relative to one another.