Q-enzyme

A three-substrate (A, B, and C), two-product (P and Q) enzyme reaction scheme in which all substrates and products bind and are released in an ordered fashion. Glyceraldehyde-3-phosphate dehydrogenase has been reported to have this reaction scheme. The steady-state and rapid equilibrium expressions, in the absence of products and abortive complexes, are identical to the ordered Ter Ter mechanism. See Ordered Ter Ter Mechanism... 

Formation of the a-( 1— 6) linkage branch points present in amylopectin and phytoglycogen is catalyzed by the branching enzyme (EC 2.4.1.18 Bourne and Peat, 1945 Hobson et al., 1950), also called the Q enzyme. 

Blennow, A., and Johnson, G. 1991. Isolation of Q-enzyme with Mr 103,000 from potato tubers. Phytochem. 30,437-444. 

Nakamura, Y and Yamanouchi, H. 1992. Nucleotide sequence of a cDNA encoding starchbranching enzyme or Q-enzyme I from rice endosperm. Plant Physiol. 99, 1265-1266. 

Yamanouchi, H., and Nakamura, Y. 1992. Organ specificity of isoforms of starch branching enzymes (Q-enzyme) in rice. Plant Cell Physiol. 33, 981-985. 

Q-Enzyme is the enzyme thought to be responsible for effecting a D-glucosyl exchange reaction in which an a-D-(l- 4)-linkage in amy-lose is converted into an a-D-(l—>6)-linkage. Probably present in all plants, the enzyme was first isolated, and later crystallized, from the potato. Purification was achieved by fractional precipitation with ethanol at low temperatures. Few properties of the crystalline enzyme were detailed, but it was found to be very heat-labile, to be stabilized to some extent by the presence of substrate, and to be activated by inorganic ions. Later experiments have shown this preparation of enzyme to be impure. "... 

In view of its possible importance in starch synthesis, surprisingly little is known about the specificity and mechanism of action of Q-enzyme. It has been suggested both that G40 is not a substrate for the enzyme, and that G40 is the minimum length for rapid action. Also, it has been reported that Q-enzyme acts more rapidly on maize amylopectin than on the amylose thereof. ... 

If ramified molecules exist in starch and in glycogen, it must be concluded that, under certain conditions, the attachment of the new D-glucose unit sometimes also takes place in the 6-position. In the case of amylopectin, the result is a polysaccharide with something like 5.5% 1,6-a-D-glucosidic linkages. Haworth, Peat and Bourne have shown that in addition to the phosphorylase discovered by Hanes (termed P-enzyme ) there is in the potato a second enzyme, termed Q-enzyme. The combined action of the two enzymes leads to the formation of branched-chain... 

To produce amylopectin, it is necessary to have a starch branching enzyme that is capable of introducing the a-(1 6) branch linkages into the linear amylose molecules synthesized by starch synthase. Such an enzyme (Q-enz)mre) was first identified in potatoes [131] and then in broad beans [132]. The potato Q-enzyme has been isolated and purified and the properties determined [133]. Q-enzyme is a glucanyltransferase and cleaves an a-jl— 4) linkage of an amylose chain, and then transfers the chain to the C6-OH of another amylose-chain, rather than being transferred and attached to the residual part of the cleaved chain [134]. It was shown that water is not involved in these branching reactions [133]. 

The continuing interest of Bourne in the chemistry of polysaccharides and associated enzymes originated from the work of Haworth and Peat directed towards the enzymic synthesis and degradation of starch. The impetus for this work was given by the discovery, made by C. S. Hanes in 1940, that a phosphorylase isolated from the potato and pea effects the synthesis, from D-glucosyl phosphate, of starch, later shown (by Haworth, Heath, and Peat) to be amylose. In his first paper (with Haworth and Peat) in 1944, Bourne described the isolation of the Q-enzyme which, in conjunction with phosphorylase, effects the conversion of D-glucosyl phosphate into the major component of whole starch, namely, amylopectin. He had discovered the Q-enzyme in a fraction discarded by previous workers. Already, the quintessence of his mind was revealed in this work meticulous attention to detail, and perception of essentials. 

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