An electron microscopy study of the quarternary structure of yeast phosphofructokinase

Homogeneous phosphofructokinase from yeast with a molecular weight of 835 000 and composed of eight subunits (four alpha and four beta subunits) was examined by electron microscopy and image computer processing. Three types of particles were seen representing different projections of the phosphofructokinase molecule. A structural model of the enzyme was developed, each of the subunits having two oblong "domains" with a length of 8 and 4.5 nm, respectively. The smaller "domain" is attached to the center of the larger one. One alpha and one beta subunit are regarded to form a heterodimer. Four heterodimers (alpha beta) are tetrahedrally assembled giving rise to point group symmetry 222.