April 17, 2013 | David F. Coppedge

Wood You Cellulose for Starch?

Cellulose is the most abundant biomolecule, but how it’s made still baffles scientists.  Soon, though, you may be able to eat it.

Two stories in the Proceedings of the National Academy of Sciences (PNAS) talked about cellulose: one, about how it’s made, and the other, about we might remake it into food.

Baffling Molecular Machine

The first paper in PNAS is trying to tease out the structure of cellulose synthase (CESA), a complex enzyme that puts together the ingredients into the strong stacks of microfibrils that make wood sturdy and hard to digest.  The team of 7 researchers said, “A 3D atomistic model of a plant cellulose synthase (CESA) has remained elusive despite over forty years of experimental effort.”  The enzyme is composed of over 500 amino acids.  Even this paper doesn’t have the structure all worked out. “Cellulose is nature’s most abundant renewable biomaterial and an important resource for production of biofuels that represent alternatives to fossil fuels,” the press release said, without revealing the details of how it works.

A press release from North Carolina State University where one of the researchers works shows it to look like a ring that braids the microfibrils.  Lecture notes from the University of Kentucky show the machine climbing a microtubule as it removes the fructose from sucrose, weaving the microfibrils of glucose into a stiff tapestry.  Box 1 of a paper from Trends in Plant Science (2012) says that 36 proteins are involved with CESA, adding, “many proteins regulate the synthesis of cellulose, either via a direct interaction… or indirectly… Mutations in any of these genes also lead to reduced cellulose content or crystallinity and cell expansion defects of roots and hypocotyls.”   With the structure becoming better known, the enzyme’s processivity (method of operation) is bound to be interesting.  The research team studied CESA within cotton fibers.  What cotton-picker or cotton-spinner of centuries past would have thought that in his or her hands was a molecular machine that would defy the understanding of 21st century scientists?

Swords to Plowshares

The second paper in PNAS describes a way to convert cellulose into starch.  Cellulose, consisting primarily of glucose in chains, has proven indigestible except by bacteria within the stomachs of some herbivores.   Cellulose and starch (made of glucose) have the same basic structure, differing only in the linkages between molecules.  Finding a way to turn that abundant biomolecule into food would be valuable to feed a growing world population.  A team at Virginia Tech found a way to do it using 5 non-natural enzymes derived from fungi, yeast and plant material.

The process yielded 30% of amylose, a linear form of starch, with the remainder available for biofuel.  It’s environmentally friendly, according to Science Daily; it doesn’t require heat, chemicals or expensive equipment, and can be upscaled for commercial production.  And because the enzymes can be recycled with magnetism, it doesn’t generate any waste.  The abstract stated, “Next-generation biorefineries based on simultaneous enzymatic biotransformation and microbial fermentation could address the food, biofuels, and environment trilemma.”  Earlier this month, PhysOrg reported on attempts to turn xylan, the world’s second most abundant biomolecule, into biofuels.  It’s more challenging because of the pentose structure of its sugars gets in the way of extracting the hexose sugars in cellulose that are more easily fermented.  The article spoke of this Department of Energy initiative in Biblical terms, likening it to turning “swords to plowshares.”

Like the old motivational speech “Acres of Diamonds” emphasized, people are surrounded by treasures if they just knew how to find them and use them.  Here is more good Darwin-free science that can actually increase our understanding (and wonder) of the natural world, and at the same time, turn that understanding into technologies that can help humanity.




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