Genetic Engineering of Starch Biosynthesis in Maize Seeds for Efficient Enzymatic Digestion of Starch during Bioethanol Production

Abstract

Maize accumulates large amounts of starch in seeds which have been used as food for human and animals. Maize starch is an importantly industrial raw material for bioethanol production. One critical step in bioethanol production is degrading starch to oligosaccharides and glucose by α-amylase and glucoamylase. This step usually requires high temperature and additional equipment, leading to an increased production cost. Currently, there remains a lack of specially designed maize cultivars with optimized starch (amylose and amylopectin) compositions for bioethanol production. We discussed the features of starch granules suitable for efficient enzymatic digestion. Thus far, great advances have been made in molecular characterization of the key proteins involved in starch metabolism in maize seeds. The review explores how these proteins affect starch metabolism pathway, especially in controlling the composition, size and features of starch. We highlight the roles of key enzymes in controlling amylose/amylopectin ratio and granules architecture. Based on current technological process of bioethanol production using maize starch, we propose that several key enzymes can be modified in abundance or activities via genetic engineering to synthesize easily degraded starch granules in maize seeds. The review provides a clue for developing special maize cultivars as raw material in the bioethanol industry.

Keywords: bioethanol; genetic engineering techniques; maize seeds; modifying starch synthesis; proteomic analysis; yeast fermentation.

Figure 1

Flow chart of the wet milling process, dry grind process and cold starch hydrolysis process for bioethanol production. Left panel, conventional protocol; right panel, GSH protocol. AMY, α-amylase; GSH, granular starch hydrolysis; GSHE, granular starch hydrolysis enzymes.

Figure 2

Starch granules in maize kernel and starch degradation during seed germination. The key enzymes involved in starch degradation in vivo were indicated. (A) Starch degradation represented within a seed. GA, gibberellic acid. (B) Scanning electron microscopy of starch granules in maize endosperm [68]. Bar = 20 μm. 1, starch granule; 2, protein body; 3, cell wall; 4, amorphous debris. (C) Scanning electron microscopy of isolated starch granules [68]. Bar = 20 μm. (D) Starch degradation pathway in vivo. AMY, α-amylase; BMY, β-amylase; ISA, isoamylase; LD, limit dextrinase; AGL, α-glucosidase.

Figure 3

Starch biosynthesis in maize endosperm. Black arrows show the conventional biosynthesis of starch: glucose-1-P is converted to ADPG, which is transported into the amyloplast and polymerized into starch. Red arrows show the oxidative pentose phosphate pathway, which provides NADPH and pentose sugars for starch synthesis. 1, sucrose synthase; 2, UDP-glucose pyrophosphorylase; 3, hexokinase; 4, AGPase; 5, phosphoglucomutase; 6, complexes including AGPase, starch synthase IIa (SSIIa), SSIII, starch branching enzyme IIb (SBEIIb), and SBEIIa; 7, glucose-6-phosphate dehydrogenase; 8, 6-phosphogluconate dehydrogenase; 9, malic enzyme; 10, transaldolase; 11, transketolase; 12, triose-P/P translocator and P/phosphoenolpyruvate translocator.