Biochemistry
Normal dent maize has two different pathways for starch formation one leading to branched chain (amylopectin) and the other to straight-chain polysaccharides (amylose).
- The amylopectin consists of chain of α-D-(1-4) and α-D-(1-6)-glucosidic linkages that form a branched molecule.
- Amylose is primarily linear with α-D-(1-4)-linked glucose residues.
The locus Wx codes for a specific starch granule-bound enzyme, NDP-glucose-starch glucosyltransferase. This specific starch synthase enzyme is responsible for amylose biosynthesis. The Wx gen catalyses the 1–4 linkage from glucose residues to amylose synthesis in the developing endosperm. This enzyme is located in the amyloplasts and is the major component of the starchbound protein in maize. Nelson showed that starch granules from wx wx wx endosperm had very low starch granule-bound glucosyltransferase activity.
When measuring if the activity of the transferase was a function of the Wx dosage in diploid and tetraploid maize, Akatsuka noticed a linear proportionality between a preparation of Wx Wx Wx and Wx Wx Wx Wx Wx Wx . Nevertheless the amylose content was the same in both types suggesting that activity of the transferase is not directly linked to the amylose content.
In maize and some other plants, there is evidence of a starch molecule that is intermediate in size to amylose and amylopectin. The intermediate fraction contains chains of (1–4)-linked alpha-D-anhydroglucose residues, but the average length of these chains and the number of chains per molecule are different from those in either amylopectin or amylose. Several researchers demonstrated the presence in normal maize starch of about 5 to 7% intermediate polysaccharides, basing their conclusions on indirect evidence from IA.
As early as in 1956, it was stated that amylopectin contained three different types of chains. In each macromolecule there is one C-chain, which carries the only reducing group. The B-chains are linked to the macromolecules linked by their potential reducing group, and may contain one or more A-chains that are similarly linked. The ratio of A-B chains (1:1 to 1,5:1) is a measure of the degree of multiple branching and is an important property describing amylopectin. Nevertheless the exact arrangement of chains within the amylopectin molecule is still not clear.
Combining the recessive mutant (wx) maize variant with other mutant as for example amylose extender (ae)maize and dull (du) maize has an effect on the amylose and amylopectine structure of the starch.
- The amylose extender waxy (aewx) starch contain 21% apparent amylose and has a lambda max. of 580 for the iodine-starch complex. The aewx outer chains are longer than those of wx mutant and fewer in number per weight of starch. In general, the aewx starch had a unique structure that is similar to the anomalous amylopectin (intermediate fraction) reported in ae starch.
- Increased dosage at the ae locus, regardless of the genotype at the wx locus, resulted in amylopectin with increased linearity.
- Short-chained amylose (approximately 100 glucose units) was observed in all ae genotypes in a homozygous Wx background.
- Amylopectin of the aewx mutants had an increased proportion of long B-chains and a decreased proportion of short B-chains compared with wx amylopectin, whereas amylopectin of the dull waxy (duwx) mutant had a decreased proportion of long B-chains and an increased proportion of short B-chains, thus confirming the novel nature of aewx and duwx amylopectin.
Read more about this topic: Waxy Corn