A-1,6 Branch linkage

Therefore the a(1 6) branching linkages were hydrolized selectively. The resulting constituting chain-length distribution was determined on a semipreparative Sephacryl S-200/S-400/S-500/S-1000 (12 -I- 55 -I- 66 -I- 135 X 1.6 cm)... 

G) amylopectin (95% a-1- 4 linkages with 5% a-1- 6 branch linkages) synthesized by N. perflava species amylosucrase. 

The amounts of amylose and amylopectin differ for starches from different botanical sources. Most so-called normal starches have 20-30% amylose and 80-70% amylopectins, respectively [29,30]. There are mutant varieties, such as waxy maize, waxy rice, and waxy potato, that are composed of 100% amylopectin. There also are the high amylose varieties, such as amylomaize-V that consists of 53% amylose and 47% amylopectin and amylomaize-Vn that is 70% amylose and 30% amylopectin, just the reverse of the normal starches. Many of the normal starches have been found to have an intermediate component that is slightly branched amylose with 0.5-3% a-(1 6) branch linkages [26,27,29,30]. 

Segment of an amylopectin molecule, showing one a-1,6 branch linkage... 

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 formation of amylopectin occurs through the action of starch branching enzyme on mal-todextrins and/or amylose to form the a-(1 6) branch linkage. Amylose or maltodextrins can be used as the substrates, but the amylose must be at a concentration of 1 mg/mL or less, as higher concentrations leads to the precipitation (retrogradation) of amylose from solution. 

Starch is a polysaccharide composed exclusively of D-glucose and is one of the three most abundant organic compounds found on Earth (cellulose and murein being the other two abundant compounds). Most starches are composed of two types of polysaccharides amylose and amylopectin. The former is a mixture of linear polysaccharides of D-glucose units linked a-1 4 to each other. The latter consists of a mixture of branched polysaccharides of D-glucose unit linked a-1 4, with 5% of a-1 6 branch linkages corre-... 

Starch phosphorylase incorporates phosphate, rather than water, across the a-1,4 linkage between the penultimate and last glucose at the non-reducing end of the polysaccharide chain. One molecule of glucose-1-phosphate is released. This enzyme attacks amylose repetitively and amylopectin can be degraded to within two or three glucose residues of a a-1,6 branch linkage. 

Methylation analysis of glycogen showed that it was an a-1 4-linked glucan with 10% a-1 6 branch linkages [123]. It is a highly water-soluble, ramified... 

Figure 10.16. Mechanism for forming a-1 6 branch linkages by potato starch branching enzyme (Q-enzyme) acting on double helical chains A, mechanism for the cleavage and transfer in a double helix B, magnification of (A), showing the reactions on a molecular level.

Reaction 2 attack of C-6 hydroxyl group of the second chain onto the enzyme-amylose chain covalent complex to form a-1,6 branch linkage... 

There is an exo-acting bacterial a-amylase, elaborated by Pseudomonas stutzeri, that exclusively produces maltotetraose from starch [10]. The enzyme hydrolyzes maltotetraose units from the nonreducing ends of the starch chains, and, like P-amylase, it will not bypass the a-1 6 branch linkages of amy-... 

Pseudomonas amyloderamosa elaborates an isoamylase that specifically hydrolyzes the a-1 -> 6 branch linkages of starch [16-20]. Another enzyme that also will hydrolyze the a-1 6 branch linkages of starch is pullulanase, which is elaborated by a strain of A. aerogenes [20]. This enzyme is a hydrolase that specifically hydrolyzes the a-1 6 linkages of the linear polysaccharide, pullulan... 

Although it will also hydrolyze the a-1 6 branch linkages of amylopectin, it hydrolyzes the a-1 6 branch linkage of chains longer than 2-3 glu-... 

Phosphorylase does not bypass a-1 6 branch linkages. It does not even re-... 

The resistant fraction gave a deep blue color with triiodide, similar to that given by amylose. Treatment of the original amylose or the resistant fragment with pul-lulanase increased its conversion to 100% (3-maltose, indicating that some of the amylose molecules had a-1 6 branch linkages that prevented complete conversion to -maltose by (3-amylase [28,29]. 

Starch branching enzyme is a glucanosyltransferase that catalyzes the cleavage of an a-1 4 linkage to give an a-1 6 branch linkage (see Chapter 10, Fig. 

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