New research: What does yeast eat to grow well? The answer is "wood"?

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

Yeast is one of the most familiar microorganisms in our lives. Baker's yeast is used for steaming buns and baking bread, and beer yeast is used for brewing beer. In addition, there are many strange yeasts, such as yeast that can "eat" methanol and produce high protein (such as Pichia pastoris), and yeast that "eats" xylose and grows wildly (such as Hansenula polymorpha).

Will new yeast with more functions be produced in the future? The answer from scientists is: Of course!

Yeast is a cell factory

Before we understand how scientists "create" more new yeast, let's first look at how yeast "works".

Although both baker's yeast and brewer's yeast belong to the category of brewing yeast, the specialty of baker's yeast is that it "eats sugar and produces gas" . The carbon dioxide produced makes steamed buns or bread porous and soft; while the specialty of brewer's yeast is that it "eats sugar and produces alcohol". Of course, it also produces a small amount of other aroma components, thus giving the wine a variety of flavors.

Dry yeast (Image source: Veer Gallery)

Let’s take the example of brewer’s yeast “eating glucose to produce ethanol”. We can regard yeast as a factory and glucose as the raw material. The raw material is transported to the factory (that is, into the yeast cells), processed in different “workshops”, and converted step by step into products - ethanol and carbon dioxide.

Yeast cells (Image source: Veer Library)

The yeasts we currently have are diverse, but we would like to have yeasts that are versatile and high performing.

If you want to have more types of yeast, the key is to transform the "factory". The method that researchers have come up with is gene editing. Change the design drawings of the yeast factory (the gene sequence of yeast cells), thereby transforming the factory layout, and then adjust the number and function of the "workshops" according to the new drawings.

Gene editing is equivalent to using a pair of scissors with built-in navigation function. It can accurately cut a gap at the corresponding position of the yeast chromosome according to human settings, and then repair the gap through the target DNA sequence, thereby achieving the effect of gene editing.

Genetic engineers artificially modify genes (Image source: Veer Gallery)

Many scientists conduct research on yeast. For example, the team of Professor Jay Keasling, a member of the U.S. National Academy of Sciences, synthesized artemisinic acid, the precursor of the antimalarial drug artemisinin, by modifying brewer's yeast. This has made an important contribution to the treatment of malaria worldwide and proved that yeast can do a lot.

Yeast: Get some glucose to eat

The yeast we mentioned in the examples above only start to work after "eating" glucose, which is yeast's favorite "food ".

However, glucose mainly comes from food, and giving glucose to yeast means that yeast will compete with humans for food. Is there any way to "feed" yeast without reducing food?

That's what scientists are considering.

In recent years, the country has encouraged the vigorous development of non-food biomass resources, including cellulose .

Although the term "lignocellulose" may sound unfamiliar, it is a very common and easily available substance that is the main structural component of wood and crop straw. It is a complex and diverse "family" composed mainly of three key elements.

Cellulose is a "big" polysaccharide composed of glucose, the main component of plant cell walls, and one of the most widely distributed and abundant polysaccharides in nature . Hemicellulose is a "polymer" composed of many different types of monosaccharides, which forms a strong fibrous network between cells and achieves a close connection between cells . Lignin is one of the components of plant cell walls, and it acts as a "scaffold" for the fibers to strengthen the structure of the entire cellulose .

Lignocellulose is a valuable natural renewable resource with broad application potential. It can be used as a raw material for biofermentation and biochemical engineering .

Yes, it can be a source of nutrition for yeast.

A strong, biodegradable and recyclable lignocellulosic bioplastic (Image source: Reference [5])

However, due to its very stable structure, it is difficult to be used directly . Therefore, it is necessary to use specific treatment methods, such as strong acid or strong alkali , to break down cellulose and hemicellulose into their basic monosaccharide components (such as glucose and xylose), which can be directly used by certain yeasts .

However, this does not mean that we can now completely abandon food and use lignocellulose for yeast cultivation. The main reason is that lignocellulose needs to be hydrolyzed into monosaccharides before it can be used by yeast, and the cost of preparing hydrolyzate is still relatively high. In addition to the main components of glucose and xylose, the hydrolyzate also contains some inhibitory components (such as furfural, furan, etc.), which have a certain inhibitory effect on yeast growth. Therefore, the use of lignocellulose hydrolyzate for microbial fermentation has not been widely used.

Killing three birds with one stone

Yeast loves to "eat" glucose the most. When glucose and xylose exist at the same time, yeast will eat glucose first and will eat xylose after the glucose is eaten up. This makes the yeast utilize lignocellulose hydrolysate more slowly .

How can yeast efficiently utilize lignocellulose hydrolysate?

Recently, the team led by Researcher Zhou Yongjin of the Chinese Academy of Sciences modified Hansenula polymorpha through genetic engineering. Without affecting the glucose digestion rate, they enhanced the yeast's absorption and digestion rate of xylose, thereby achieving the simultaneous utilization of glucose and xylose, improving the utilization efficiency of lignocellulose hydrolysate by Hansenula polymorpha, and synthesizing fatty acids and 3-hydroxypropionic acid by modifying Hansenula polymorpha.

The Chinese Academy of Sciences has achieved efficient synthesis of fatty acids and 3-hydroxypropionic acid from lignocellulose biorefining (Image source: Researcher Zhou Yongjin's team at the Chinese Academy of Sciences)

Compared with fatty acids that can form oils, the term 3-hydroxypropionic acid is relatively unfamiliar to us.

In fact, 3-hydroxypropionic acid can be used as a raw material for many chemicals. For example, 3-hydroxypropionic acid can be polymerized into poly-3-hydroxypropionic acid. As a degradable plastic , if used in large quantities, it can alleviate "white pollution" ; 3-hydroxypropionic acid can also be dehydrated to form acrylic acid, which can be further prepared into acrylic resin . Acrylic resin is the main component of decorative coatings and paints , which are closely related to our lives.

Schematic diagram of Hansenula polymorpha using lignocellulose as raw material to efficiently synthesize fatty acids and 3-hydroxypropionic acid (Image source: Researcher Zhou Yongjin's team at the Chinese Academy of Sciences)

Chinese scientists produce fatty acids and 3-hydroxypropionic acid through cellulose biorefining. Compared with traditional glucose biorefining, cellulose, as a non-food biomass, can avoid "competing with people for food" and reduce air pollution caused by direct burning of straw and waste wood. It can be said to "kill three birds with one stone."

Conclusion

Yeast cell factories have shown extremely high application value in scientific research. Scientists have transformed yeast through precise gene editing technology and synthesized a series of high value-added products, such as biofuels, fine chemicals, spices and food additives. However, traditional biorefining using glucose as raw material may face the dilemma of competing with people for food. In order to cope with this dilemma, the modified yeast can effectively utilize glucose and xylose in lignocellulose hydrolysate, and also respond to the country's call to develop non-food biomass.

Experimental studies have demonstrated the feasibility of using glucose and xylose to synthesize fatty acids and 3-hydroxypropionic acid by modifying Hansenula polymorpha, proving the great potential of lignocellulose biorefining. With the unremitting efforts of the scientific and industrial communities, lignocellulose will be widely used in the production of more products in the future, providing strong support for the sustainable development of my country's biochemical industry.

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