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Xingye Yu, a graduate student in chemical engineering, and Professor Chaitan Khosla examine a culture of E. coli bacteria. Photo: L.A. Cicero
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When it comes
to making biodiesel cheaply and efficiently enough to be commercially feasible,
E. coli may prove to be
"the little bacterial engine that could," say Stanford University
researchers.
Biodiesel can
be made from plant oil or animal fat—usually the former. Used cooking oil from
restaurants is common, but for biodiesel to contribute significantly to
reducing fossil fuel use, there needs to be a way to mass produce it from
plant-derived raw materials. The problem is that synthesizing biodiesel is
complicated. That is where E. coli
comes in.
The bacteria,
often discussed in terms of the human digestive tract, also act as a catalyst
in generating biodiesel by converting inexpensive sugars into fatty acid
derivatives that are chemically similar to gasoline.
But E. coli's natural conversion capability
is not up to snuff, commercially speaking, and researchers tinkering with its
internal machinery have yet to boost its capability enough to cross the
commercial threshold.
So Chaitan Khosla, a Stanford professor of chemistry and of chemical
engineering, decided to investigate whether there might be a natural limit that
holds back E.
coli's conversion capabilities. In other words, does the basic
catalytic engine in E. coli have enough horsepower to do the job at the
needed scale?
A powerful engine
"The good news is that the engine that makes fatty acids in E. coli is incredibly powerful,"
Khosla says. "It is inherently capable of converting sugar into fuel-like
substances at an extraordinary rate. The bad news is this engine is subject to
some very tight controls by the cell."
It turns out
that like any high performance engine, the catalytic process in E. coli can only attain peak efficiency
when all the controls are tuned just right. The research is described in a
paper published in Proceedings of the
National Academy of Sciences. Khosla is a coauthor of the paper,
which is available online.
Scientists
don't yet understand how all the cellular controls operate. It will require a
deeper understanding of the biochemistry of E.
coli than they have now to figure that out, Khosla says. But his
research team is making progress homing in on the most promising part of the
conversion process, thanks in part to a new approach they employed in their
analysis.
The
researchers managed to isolate all the enzymes and other molecular participants
involved in the process that produces fatty acids in E. coli and assemble them in a test
tube for study.
"We
wanted to understand what limits the ability of E. coli to process sugar into oil. The question we were
asking is analogous to asking what limits the speed of my Honda to 150 mph and
no faster?" Khosla says. "The most direct and powerful way to figure
it out is to pull the biosynthetic engine out of the cell and put it through
its paces in a test tube."
By doing so, the team was able to study how the enzymes involved in fatty
acid biosynthesis performed when they were free from other cellular influences.
That was critical to their analysis, because the products in question, fatty
acids, are essentially soap, Khosla says, and too much of them would hurt the
bacteria. That is why E. coli has developed some very elaborate and
effective ways to contain the amount of fatty acid biosynthesis inside the cell.
Precursor to biodiesel
The
fatty acids can't be pumped directly into your gas tank—cars and trucks won't
run on soap, after all—but they are an excellent precursor to biodiesel.
Biodiesel has
so far lagged behind ethanol as a means of cutting fossil fuel use in vehicles
because ethanol is easier and cheaper to make. But biodiesel has a higher
energy density and lower water solubility than ethanol, which offer significant
advantages.
"It is
closer in chemical properties to a barrel of oil from Saudi Arabia
than any other biologically derived fuel," Khosla says. Thus it could
easily be blended into diesel and gasoline, or used alone as a bona fide
transportation fuel.
If researchers can figure out how to manipulate the cellular means of production
in E. coli,
biodiesel could be made cheaply enough that the little engine of E. coli
could end up powering a lot of larger engines at far less cost to the
environment than with fossil fuels.
SOURCE
