Glucose-fructose isomerisation

The two largest scale industrial processes utilising immobilised enzymes are the hydrolysis of benzyl penicillin by penicillin acylase and the isomerisation of glucose to a glucose-fructose mixture by immobilised glucose isomerase. The immobilisation techniques used in general may be broadly categorised as ... 

The conversion of ball-milled cellulose, glucose and fructose into micro-ciystalline lactic acid and 5-hydro g methylfurfural in water, catalysed by lead(ii) nitrate, has been reported (Scheme 22.5). The process involves a multistep cascade including the hydrolysis of cellulose to glucose, the isomerisation of glucose to fructose, retro-aldol fragmentation of fructose to trioses and their subsequent conversion to lactic acid lead(ii) catalyses both the conversion of glucose to fructose and the multistep cascade from fructose to lactic acid. The capacity of lead(ii) to be chelated by... 

Glucose 6-phosphate is then isomerised to form fructose 6-phosphate. 

Glucose syrups have been used in the food industry for a long time. Fructose is significantly sweeter than glucose. No effective chemical isomerisation methods are possible, and other sources of fructose, for instance by the hydrolysis of inulin, are not yet performed on large scale. Therefore an enzyme isomerisation technology has been developed (Jensen and Rugh, 1987 White, 1992 Pedersen, 1993). 

The discovery of a non-cofactor deperrdent errzyme (glucose isomerase) that wottld isomerise glucose into fructose (see Figrrre 4.1). 

Isomerised Glucose, also known as Isoglucose or High Fructose... 

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions." In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion." With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

Zhang, Y, Hidajat, K., and Ray, A. (2007) Modified reactive SMB for production of high concentrated fructose syrup by isomerisation of glucose to fructose. Biochem. Eng. J., 35, 341-351. 

The term isomerase is used to describe an enzyme which effects an isomerisation process. This is illustrated by the equilibrium which can be set up between fructose-6-phosphate and glucose-6-phosphate (11.32). These two compounds are both found in yeasts and animal tissues. Another example is provided by the equilibrium between glucose-l-phosphate and glucose-6-phosphate (11.33). 

The conversion of D-glucose (17) into D-fructose (9) by a microbial enzyme (Scheme 5) was first reported in 1957 when Marshall and Kooi found glucose isomerase activity in cell-free extracts of Pseudomonas hydrophila (91. This enzymatic activity was enhanced in the presence of arsenate. Soon thereafter, other arsenate-requiring enzymes were isolated from Aerobacter sp. as well as Escherichia freundii [10]. Enzymes required arsenate when D-glucose or D-fructose was the substrate but not when the corresponding 6-phosphates 11 and 12 were offered. Purification of the arsenate-dependent principle component from Escherichia intermedia allowed the conclusion that the enzyme was a glucose 6-phosphate isomerase (EC 5.3.1.9) that was able to isomerise free D-glucose when it was complexed with arsenate [11]. 

In a seminal contribution, Bock and co-workers conducted mechanistic studies on soluble as well as immobilised xylose isomerase from Streptomyces murinus sp. (Sweetzyme Q from NOVO) employing NMR-spectrometric methods. These studies confirmed the lack of deuterium exchange with solvent molecules of D-glucose and D-fructose regiospecifically C-deuterated at C-1 and C-2 [31]. It is noteworthy that they found the (IS)-diastereomer of D-(l- H)fruc-tose (29) was formed with perfect selectivity by isomerisation of D-(l- H)glu-cose (28) and so was the corresponding (IR)-epimer (31) when D-(2- H)glu-cose (30) was employed as the substrate (Scheme 6). 

Subsequently, Card and co-workers found a conversion rate of 10-15% for the isomerisation of 6-deoxyfluoro-D-glucose (61) into the corresponding d-fructose derivative 62 (Scheme 22) [77]. 

The enzymatic conversion of 6-azido-6-deoxy-D-glucose into its D-fructose isomer was utilised as a key step in a five-step synthesis from sucrose [80] of the natural product and powerful D-mannosidase inhibitor 1-deoxymannojirimycin (l,5-dideoxy-l,5-imino-D-mannitol, 66) [81]. Subsequently, it was shown that the immobilised enzyme from Streptomyces murinus sp. (Sweetzyme T from Novo A/S) was able to isomerise D-glucose derivatives with modifications at C-3 and C-6 such as 6-azido-6-deoxy-3-0-methyl-D-glucose (67) as well as the corresponding 3-deoxy derivative 69 (Scheme 24) [64,82]. 

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