Diol dehydrase

Mechanism (1), first suggested almost three decades ago [150], continues to be the most invoked explanation of the role of the enzyme in promoting homolysis. Cleavage of the C—Co bond of sterically hindered alkylcobalamins (e.g., neopentylcobalamin) was markedly increased by diol dehydrase [72], Such cobal-amins do not function as coenzymes but convert to enzyme-bound hydroxocobal-amin in stoichiometric first-order reactions. The strong competitive inhibition by AdoB 12 indicates that labilization occurs at the active site of the enzyme and is suggested to be caused by a steric distortion of the corrin ring. 

Structure-Function Relationship of Vitamin Bi2 Coenzyme (Adenosylcobalamin) in the Diol-Dehydrase System... 

Enzymological Properties and Mechanism of Action of Diol Dehydrase... 

Protein-chemical Properties of Apoenzyme. Adenosylcobalamin-dependent diol dehydrase was discovered and isolated first by Abeles and co-workers (3, 4) in the cells of Klebsiella pneumoniae (formerly known as Aerohacter aero genes) ATCC 8724 grown without aeration in a glycerol or glycerol-1,2-propanediol medium. This enzyme catalyzes the conversion of 1,2-propanediol, 1,2-ethanediol, and glycerol to propionaldehyde, acetaldehyde, and j3-hydroxypropionaldehyde, respectively (4, 5). Adenosylcobalamin and K+ or other monovalent cations of a similar size are required for catalysis. Recently, the au-... 

Interaction with Adenosylcobalamin. It has been considered generally that adenosylcobalamin or its analogs binds to the apoprotein of diol dehydrase or other adenosylcobalamin-dependent enzymes almost irreversibly (4). However, we found that the holo-enzyme of diol dehydrase was resolved completely into intact apoen-zyme and adenosylcobalamin when subjected to gel filtration on a Sephadex G-25 column in the absence of K+ (9, 10). Among the inactive complexes of diol dehydrase with irreversible cobalamin inhibitors, those with cyanocobalamin and methylcobalamin also were resolved upon gel filtration on Sephadex G-25 in the absence of both K+ and substrate, yielding the apoenzyme, which was reconstitutable into the active holoenzyme (II). The enzyme-hydroxocobalamin complex, however, was not resolvable under the same conditions. The enzyme-cobalamin complexes were not resolved at all by gel filtration in the presence of both K+ and substrate. When gel filtration of the holoenzyme was carried out in the presence of K+ only, the holoen-... 

Mechanism of Action. Diol dehydrase is one of the enzymes whose mechanism of action has been studied extensively (13, 14). It has been established that in general the rearrangement reactions catalyzed by adenosylcobalamin-requiring enzymes involve the migration of a hydrogen from one carbon atom of substrate to an adjacent carbon atom in exchange for a group X that moves in the opposite direction. In the case of the diol-dehydrase reaction, X represents a... 

Scheme 1. Mechanism of action of diol dehydrase RCH2— = adeno-syl [Co] = cobalamin SH = substrate PH = product...

Succinyl and 5 -0-succinyl derivatives of cyanocobalamin were not inhibitory for diol dehydrase even when enzyme was prein-... 

Several analogs in which 5,6-dimethylbenzimidazole is replaced by different heterocyclic bases have been tested for coenzyme activity with diol dehydrase by Abeles and Lee (3) and Kamikubo and coworkers (28, 29, 30). It is noteworthy that relative activity of these analogs do not differ so much as do their apparent Km values (Table III). This suggests that the base moiety is important for the recognition of the coenzyme by the enzyme molecule, but not directly for the catalytic action. 

Friedrich et al. have reported that the analog with 1,1-dimethyl-l-amino-2-ethanol in place of 1-amino-2-propanol is inactive as a coenzyme in the diol-dehydrase system, although it is bound to the enzyme as tightly as is cyanocobalamin (31). 

Figure 4. Affinity chromatography of diol dehydrase on the adenosyl form of II (37). About 1 unit of enzyme was applied to 1 mL of packed corrinoid gel in 0.1 M potassium phosphate buffer (pH 8.0) containing 2% 1,2-propanediol, in a total volume of 2 mL. Affinity chromatography was carried out as described in the text. Two-milliliter fractions...

FIGURE 4. The eleven adenosylcobalamin-dependent rearrangements so far described. Notes D-a-lysine [L-p-lysine] 5,6-aminomutase catalyzes similar rearrangements on two substrates diol dehydrase and glycerol dehydrase have over-lapping substrate specificity. 

DxHxxG -methyl-H4-methanopterin Diol dehydrase Corrinoid/Fe-S... 

Diol dehydrase comprises three subunits and adopts a (aPy)2 structure. The a and y subunits are tightly associated with each other as an (ay)2... 

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