ELECTRON-MICROSCOPIC ANALYSIS AND BIOCHEMICAL-CHARACTERIZATION OF A NOVEL METHANOL DEHYDROGENASE FROM THE THERMOTOLERANT BACILLUS-SP C1

Methanol dehydrogenase from the thermotolerant Bacillus sp. C1 was studied by electron microscopy and image processing. Two main projections can be distinguished: one exhibits 5-fold symmetry and has a diameter of 15 nm, the other is rectangular with sides of 15 and 9 nm. Subsequent image processing showed that the 5-fold view possesses mirror symmetry. The rectangular views can be divided into two separate classes, one of which has 2-fold rotational symmetry. It is concluded that methanol dehydrogenase is a decameric molecule, and a tentative model is presented. The estimated molecular weight is 430,000, based on a subunit molecular weight of 43,000. The enzyme contains one zinc and one to two magnesium ions per subunit. N-terminal amino acid sequence analysis revealed substantial similarity with alcohol dehydrogenases from Saccharomyces cerevisiae, Zymomonas mobilis, Clostridium acetobutylicum, and Escherichia coli, which contain iron or zinc but no magnesium. In view of the aberrant structural and kinetic properties, it is proposed to distinguish the enzyme from common alcohol dehydrogenases (EC 1.1.1.1) by using the name NAD-dependent methanol dehydrogenase

THE INTERHEXAMERIC CONTACTS IN THE 4-HEXAMERIC HEMOCYANIN FROM THE TARANTULA EURYPELMA-CALIFORNICUM - A TENTATIVE MECHANISM FOR COOPERATIVE BEHAVIOR

Arthropod hemocyanins (Hcs) are regular assemblies of l, 2, 4, 6 or 8 hexameric protein molecules. They transport oxygen and can bind it in a cooperative manner. The hexameric X-ray structures of Panulirus interruptus (spiny lobster) and of Limulus polyphemus (horseshoe crab) subunit II He were solved recently by the groups of Hol (Groningen, The Netherlands) and Magnus (Cleveland, U.S.A.). They related cooperativity to a rotational movement of a domain within a subunit and of the two trimers mutually inside the hexamer.In our study a model was derived for the structure and related to the function of the four-hexameric He from the tarantula Eurypelma californicum by combining data from electron microscopy and image processing, from the X-ray diffraction studies mentioned earlier and from amino acid sequence studies. Interhexameric contacts were determined at the level of secondary structure elements and in some cases of single amino acids. Loops, undefined in the X-ray structures of the hexamers, were often involved in these contacts. In one case the contact was formed between four parallel β-helices, two from each hexamer. Based on these findings a mechanism is proposed for the transmission of cooperativity between the hexamers, in which the concept of "helical friction" plays a key role