Proteins had been completely ignored as a potential biomaterial because they’ve been thought of mainly as food. But in fact, there are a lot of different proteins that cannot be used as food. These proteins were overlooked as a resource for fuel or for chemicals because people did not know how to utilize them or how to grow them. We’ve solved these problems.

—Professor James C. Liao, senior author

This research is the first attempt to utilize protein as a carbon source for energy production and biorefining. To utilize protein as a carbon source, complex cellular regulation in nitrogen metabolism had to be rewired. This study clearly showed how to engineer microbial cells to control their cellular nitrogen metabolism.

—Kwang Myung Cho, co-author

In nutrient-rich conditions, proteins are the most abundant component in fast-growing microorganisms. The accumulation rate of proteins is faster than that of any other raw materials, including cellulose or lipids. In addition, protein does not have the recalcitrance problems of lignocellulose or the de-watering problem of algal lipids. Protein biomass can be much more easily digested to be used for microorganisms than cellulosic biomass, which is very difficult to break down. Further, cellulose and lipids don’t contribute to the process of photosynthesis; proteins are the major component of fast-growing photosynthetic microorganisms.

The challenge in protein-based biorefining, the researchers say, lies in the difficulties of effectively converting protein hydrolysates to fuels and chemicals.

Microorganisms tend to use proteins to build their own proteins instead of converting them to other compounds. So to achieve the protein-based biorefining, we have to completely redirect the protein utilization system, which is one of the most highly regulated systems in the cell.

—Yi-xin Huo, a UCLA postdoc and lead author

Liao’s team created an artificial metabolic system to dump reduced nitrogen out of cells and tricked the cells to degrade proteins without utilizing them for growth. They applied metabolic engineering to generate Escherichia coli that can deaminate protein hydrolysates, enabling the cells to convert proteins to C₄ and C₅ alcohols at 56% of the theoretical yield.

We accomplish this by introducing three exogenous transamination and deamination cycles, which provide an irreversible metabolic force that drives deamination reactions to completion. We show that Saccharomyces cerevisiae, E. coli, Bacillus subtilis and microalgae can be used as protein sources, producing up to 4,035 mg/l of alcohols from biomass containing ~22 g/l of amino acids. These results show the feasibility of using proteins for biorefineries, for which high-protein microalgae could be used as a feedstock with a possibility of maximizing algal growth and total CO₂ fixation.

Liao’s team recycles the nitrogen from the amino group back for growth of the organism—in other words, a form of self-fertilization.

Today, nitrogen fertilizers used in agriculture and biofuel production have become a major threat to many of the world’s ecosystems, and the nitrogen-containing residuals in biofuel production can eventually turn into nitrous oxide, which is about 300 times worse than CO₂ as a greenhouse gas. Our strategy effectively recycles nitrogen back to the biofuel production process, thus approaching nitrogen neutrality. Growing algae to produce protein is like putting the interest back into the principal.

—James Liao

According to Liao’s team, the culture area needed to produce 60 billion gallons of biofuels (30% of the United States’ current transportation fuel) based on the new technology could be as little as 24,600 square kilometers—equivalent to 1.9% of the agricultural land in the US.

Developing large-scale systems is our next step. Harvesting of the protein biomass economically is a bottleneck of advancing our technology.

—Yi-xin Huo

The research was partially supported by the UCLA–Department of Energy Institute for Genomics and Proteomics.

Resources

• Yi-Xin Huo, Kwang Myung Cho, Jimmy G Lafontaine Rivera, Emma Monte, Claire R Shen, Yajun Yan and James C Liao (2011) Conversion of proteins into biofuels by engineering nitrogen flux. Nature Biotechnology doi: 10.1038/nbt.1789