Enzymes aldonic acids

We have devised a very simple procedure for the preparative synthesis of various aldonic acids from the corresponding aldoses. This green chemistry process takes advantage of the availability of cheap, robust industrial enzymes. 

Comparable to the situation for the sialic acid and KDO lyases (vide supra), sets of stereochemically complementary pyruvate lyases are known, e,g. in Pseudomonas strains, which act on related 2-keto-3-deoxy-aldonic acids [112]. The enzymes cleaving six-carbon sugar acid phosphates—the KdgA and 2-keto-3-deoxy-6-phospho-D-galactonate (20) aldolases (KDPGal aldolase EC 4.1.2.21) [139] — are typified as class I enzymes, whereas those acting on non-phosphorylated five-carbon substrates — 2-keto-3-deoxy-L-arabonate (21) (KDAra aldolase EC 4.1.2,18) [140, 141] and 2-keto-3-deoxy-D-xylonate (22)... 

Among the phosphorylated analogues, the aldonic acids are better enzyme inhibitors than the corresponding alcohols, and the alcohols are better inhibitors than the corresponding aldehydes. This may reflect the acid and alcohol molecules ability to mimic the postulated enediol intermediate geometry. 

With glucose, chemical oxidation of C-l occurs with the open-chain form, whereas enzyme-catalyzed oxidation of C-l occurs with the ring form of the sugar. The aldonic acid (gluconic acid) that is formed is an equilibrium mixture of the free acid (open chain) and the 5-lactone (ring). 

Fig. 6.— Pathway for the Enzymic Formation of 3-Deoxyulosonic Acids by the Dehydration of Aldonic Acids.

Cellobiose Oxidases.—An extracellular enzyme which utilizes molecular oxygen to oxidise cellodextrins to the corresponding aldonic acids has been isolated from culture filtrates of the white rot fungus Sporotrichum pulveru-lentum. This enzyme, named cellobiose oxidase, was purified by classical techniques and was shown to be a glycoprotein (mol. wt. 9.3 x 10 ). The u.v. spectrum of the enzyme was characteristic of a haemoprotein, there being approximately one flavin component per enzyme molecule. The possible role of this complex enzyme in cellulose degradation was discussed. 

Xylonic acid, cf. Figure 5.3, is a sugar acid that can be obtained by the complete oxidation of xylose. It has been attempted to utilize xylose by converting it to an aldonic acid (19). Xylose was converted to xylonic acid by using commercial glucose oxidase enzyme, palladium catalysis, and microbial bioconversion. 

Aldonic acids usually yield crystalline lactones, amides and other derivatives and have proved of value in the characterisation of sugars. No appreciable amounts of aldonic acids have been found in plants or animals, although certain bacteria are capable of oxidising glucose and other aldoses to the corresponding aldonic acids. Lactones have proved to be powerful inhibitors in certain enzymic reactions involving substances having structures related to the inhibitors . 

During the course of studies on the microbial production of chiral intermediates for D-pantothenic acid [2,134,135], Shimizu and co-workers found that several micro-organisms, such as Fusarium, Brevibacterium and so on, produce a novel enzyme that catalyzes the hydrolysis of aldonate lactones or aromatic lactones [136, 137],... 

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