230
G. L. CANTONI
Where the onium compound is thiamine, DPN, or nicotinamide riboside,
the enzymes involved while specific for the donor compounds, thiamine
or DPN respectively, exhibit a broad substrate specificity for the acceptor molecule; furthermore they also are able to catalyze Reaction 23
which is known as a base exchange reaction.
R—CH 2 —B + B'
R—CH 2 —B' + B
(23)
Classical examples of such base exchange reactions are the exchange
reactions between free nicotinamide and the nicotinamide moiety of
DPN (196), and between free pyridine and the thiazole moiety of thiamine (197). Although a single onium-compound-specific enzyme maycatalyze both the transalkylation and base exchange reactions, these
reactions differ in two respects. In the first place, in the base exchange
reaction the energy-rich onium bond is preserved and an onium compound with a different base is formed. Thus, for instance, in the exchanges catalyzed by a number of DPNases from various sources the
nicotinamide moiety bound through a pyridinium linkage in DPN may
be exchanged with another nicotinamide (196) or with a nicotinamide
analog, such as acetyl pyridine, isonicotinic hydrazide, or others (198).
Likewise in the thiaminase reaction enzymes from various sources, such
as shellfish, fish liver, and bacteria, will catalyze the exchange of the
thiazole moiety of thiamine with a pyridine moiety resulting in the
formation of a pyridinium compound from a thiazolium and vice versa
(199). On the other hand in the transalkylation reaction proper the
onium bond is lost and H
+ is formed as for instance in the Reaction 24
which describes the synthesis of the 4-amino-5-carboxamideimidazole
ribotide analog of DPN (200).
A second difference, which is in reality a consequence of the first, is
the lack of reversibility of Reaction 22.
A further point requires clarification: generally enzymes catalyzing
base exchange reactions are also capable of effecting a hydrolysis or
phosphorolytic cleavage of the donor compound, that is to say they can
utilize H 2 0 or orthophosphate (194) as the alkyl acceptor. Thus
DPNases from various biological sources cleave DPN with the formation of free nicotinamide and ARPPR, which symbolizes the adenine
ribose and phosphate moieties of DPN, and likewise the thiaminase of
Bacillus aneurinolyticus catalyzes the hydrolytic splitting of thiamine
into free thiazole and the hydroxymethyl pyridine moiety (201).
All these observations can be unified by the formulation shown in Fig.
16 which is modeled after earlier schemes.
A more detailed analysis of the relationship between base exchange
and hydrolytic, or phosphorolytic, reactions is possible in the case of
G. L. CANTONI
Where the onium compound is thiamine, DPN, or nicotinamide riboside,
the enzymes involved while specific for the donor compounds, thiamine
or DPN respectively, exhibit a broad substrate specificity for the acceptor molecule; furthermore they also are able to catalyze Reaction 23
which is known as a base exchange reaction.
R—CH 2 —B + B'
R—CH 2 —B' + B
(23)
Classical examples of such base exchange reactions are the exchange
reactions between free nicotinamide and the nicotinamide moiety of
DPN (196), and between free pyridine and the thiazole moiety of thiamine (197). Although a single onium-compound-specific enzyme maycatalyze both the transalkylation and base exchange reactions, these
reactions differ in two respects. In the first place, in the base exchange
reaction the energy-rich onium bond is preserved and an onium compound with a different base is formed. Thus, for instance, in the exchanges catalyzed by a number of DPNases from various sources the
nicotinamide moiety bound through a pyridinium linkage in DPN may
be exchanged with another nicotinamide (196) or with a nicotinamide
analog, such as acetyl pyridine, isonicotinic hydrazide, or others (198).
Likewise in the thiaminase reaction enzymes from various sources, such
as shellfish, fish liver, and bacteria, will catalyze the exchange of the
thiazole moiety of thiamine with a pyridine moiety resulting in the
formation of a pyridinium compound from a thiazolium and vice versa
(199). On the other hand in the transalkylation reaction proper the
onium bond is lost and H
+ is formed as for instance in the Reaction 24
which describes the synthesis of the 4-amino-5-carboxamideimidazole
ribotide analog of DPN (200).
A second difference, which is in reality a consequence of the first, is
the lack of reversibility of Reaction 22.
A further point requires clarification: generally enzymes catalyzing
base exchange reactions are also capable of effecting a hydrolysis or
phosphorolytic cleavage of the donor compound, that is to say they can
utilize H 2 0 or orthophosphate (194) as the alkyl acceptor. Thus
DPNases from various biological sources cleave DPN with the formation of free nicotinamide and ARPPR, which symbolizes the adenine
ribose and phosphate moieties of DPN, and likewise the thiaminase of
Bacillus aneurinolyticus catalyzes the hydrolytic splitting of thiamine
into free thiazole and the hydroxymethyl pyridine moiety (201).
All these observations can be unified by the formulation shown in Fig.
16 which is modeled after earlier schemes.
A more detailed analysis of the relationship between base exchange
and hydrolytic, or phosphorolytic, reactions is possible in the case of