absent, there must be a suspicion that the preparations were all contaminated with this enzyme.
Benzophenone synthase (E.C. 126.96.36.199)
Centaurium erythraea enzyme, optimum pH 7.5, acts on m -hydroxybenzoyl CoA and malonyl CoA to form 2,3',4,6-tetrahydroxybenzophenone [J35].
Aspergillus parasiticus converts hexanoate or pentanoate into norsolorinate, a polyphenolic anthraquinone, which is a postulated precursor of aflatoxin Bi. With pentanoate, an additional reaction product with a 5-oxopentane side-chain, instead of a 6-oxohexane side-chain, has been detected; if 6-fluorhexanoate is used, 6'-fluoronorsolorinate is formed [H675].
Humulus lupulus enzyme, found in cone glandular hairs, is a homodimer, monomeric molecular weight 45 000 and pI 6.1; the amino acid sequence has been determined. It utilizes one mol of isovaleryl CoA, and three mol of malonyl CoA to form the aromatic nucleus. Replacement of the former with isobutyryl CoA yields the corresponding isobutyrophenone [K175].
Penicillium patulum enzyme is a tetramer, molecular weight 750 000. It condenses acetyl CoA, and requires NADPH [G751].
Macrophoma commelinae converts a series of 2-pyrones into the corresponding benzoates, usually in good yield. The carbonyl group becomes the benzoate carboxyl, and the 3, 4, 5 and 6-substituents on the pyrone become the 2, 3, 4 and 5-substituents on the benzoate [E766].
This reaction is catalyzed by peroxidases (E.C. 188.8.131.52) from peanut, with peroxide as co-substrate. Four isozymes are found with pH optima at 6, 6.4, 8 and 8 [A2519].
Salutaridine synthase (E.C. 184.108.40.206)
Papaver somniferum enzyme, probably a P450 that requires oxygen and NADPH, is found in root, shoot and capsules, but not in latex. The enzyme acts on (R)- (but not (S)-) reticuline, and the reaction involves the formation of a 6-membered carbon ring by linkage between the two aromatic nuclei, one of which is converted into a cyclohexadienone system [K759].
This reaction has been detected in rat liver cytosol, with 5-methychrysene as substrate [G118].
1.3 Formation of heterocyclic ring systems
Indole-3-glycerol-phosphate synthase; (E.C. 220.127.116.11)
1-(o-Carboxyphenylamino)-1-deoxyribulose-5-phosphate 0 indole-3-glycerolphosphate
The Bacillus subtilis enzyme, molecular weight 23 500, differs from N-(5'-phosphoribosyl) anthranilate isomerase (E.C. 18.104.22.168) [B97].
Mushroom tyrosinase, with peroxide, acts on hordenine with an optimum at pH 6.7 to form a compound whose spectra indicates that the product is N,N-dimethylindoliumolate, presumably via a quinone [K374].
Chalcone-flavanone isomerase (chalcone isomerase; E.C. 22.214.171.124)
Grapefruit enzyme acts on chalcone-4?-neohesperosides with a free, unhindered 4-hydroxyl group. Other structural features required for substrate activity are the presence of either 2,6-dihydroxy or 2-hydroxy-4-methoxy groups. It is reversibly inhibited by cyanide but not by azide, EDTA, Hg2+ or p -chloromercuribenzoate [A2518]. Tulipa petal enzyme is cytosolic [A2523].
There are many publications on chalcone synthase in which this activity is part of the reaction system, and its requirement is implicit in the formation of all flavonoids.
This involves two enzymes in the later part of the reaction sequence, 6,7-dimethyl-8-ribityllumazine synthase and riboflavin synthase (E.C. 126.96.36.199):
2,4(1H, 3H)-pyrimidinedione + (3S)-3,4-dihydroxy-2-butanone-4-phosphate 0 6, 7-dimethyl-8-ribityllumazine
6,7-Dimethyl-8-ribityllumazine 0 riboflavin + 5-amino-6-ribitylamino-2,4(1H, 3H)-pyrimidinedione
Two synthases in Bacillus subtilis have molecular weights of 70 000 and 1 000 000. The smaller (more active) molecule appears to be a homotrimmer; the larger molecule is composed of one of the above molecules as well as about 60, possibly identical, polypeptide chains of a different type [B315]. Further studies on the larger molecule have shown that it is a complex of an icosahedral capsid of 60 b units surrounding a core of three a units. The b units catalyze the first reaction, and the a units the second reaction. In the first reaction the natural (S)-butanone can be replaced by the (R )-isomer; the latter reacts at 1/6 of the rate for the (R )-isomer. The second reaction involves a dismutation, which forms both riboflavin and the substrate for the first reaction [H398].
Formyltetrafolate cyclo-ligase (E.C. 188.8.131.52)
Sheep liver enzyme, optimum pH about 4.8, forms N5,10-methenyltetrahydrofolate with hydrolysis of the ATP co-substrate to ADP. It is inhibited by thiol-binding reagents [K826].
E. coli enzyme, optimum pH 6.3-6.4, requires FADH2 to form 5-methyltetrahydrofolate [K851].
Methylenetetrafolate dehydrogenase (NADP+)
Calf thymus enzyme, optimum pH 6.5 is very unstable, and it is protected by thiols and glycerol. The reaction is reversible, forming N5,10-methenyltetrahydrofolate preferentially [K935].
Methylenetetrafolate dehydrogenase (NAD+) (E.C. 184.108.40.206)
Clostridium formicoaceticum enzyme is a homodimer, molecular weight 60 000 and Stokes radius 3.32nm. The reaction forms 5,10-methenyltetrahodrofolate, activation energy 8.5 kcal/mol; NADP+ is not an alternative co-substrate [K928].
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