From: Challenges and advances in the heterologous expression of cellulolytic enzymes: a review
Organism | Example | Protein expression | Advantages | Disadvantages | Expression challenges |
---|---|---|---|---|---|
Homologous cellulase production systems | Â | Â | |||
Fungi | Trichoderma reesei | 14 000 to 19 000 mg/l crude enzyme solution [79] | Native system | Enzyme mix cannot be tailored for different biomass substrates | Special culturing conditions required |
Protein secretor | Comparably high production costs | ||||
High protein yield | |||||
Bacteria | Bacillus subtilis (gram positive) | Â | Inducible and auto-inducible expression possible | Rich growth medium required as a carbon source, leading to increased costs | Inducible systems more efficient but significantly more expensive |
Easy to modify genetically | |||||
Protein secretor | |||||
 | Clostridium thermocellum (gram positive) |  | Native system | Low protein yield | Special culturing conditions required |
Cellulosome producing | High production costs | ||||
Transient and stable transformation | Unwanted byproducts | ||||
Heterologous cellulase production systems | Â | Â | |||
Bacteria | Escherichia coli (gram negative) | 11.2 to 90 mg/l purified enzyme solution [79] | Industrially used, common system | Thick outer membrane restricts protein secretion (poor secretion) | Degradation of linker sequences in multi-domain cellulases |
Well-characterized genetics | Formation of inclusion bodies | ||||
Many commercially available strains and vectors | Frequently incorrect transportation across the outer membrane | ||||
Easily to modify for example for protein engineering | Â | Decreased specific activity of the cellulase can occur | |||
Yeast | Saccharomyces cerevisiae | Approximately 1 000 mg/l crude enzyme solution [79] | Protein secretor | Hyperglycosylation | Inducible systems are highly efficient but can be expensive |
Surface display possible | Expression rates lower than native systems | Increased episomal gene copy numbers leads to higher protein yields but a constant selection is necessary | |||
Industrially used, common system | |||||
Plants | Nicotiana tabacum | Up to 40% of total soluble protein, depending on the subcellular targeting inside the plant cell [79] | Cheap protein production | Transport of genetic information via pollen (if not transplastomic) | Possible glycosylation effects |
Easy transformation | Long transformation procedure | Subcellular targeting inside the plant cell very important for expression efficiency | |||
Well-characterized genetics | Â | Possible effects on plant growth behaviour | |||
Protein and biomass | |||||
Production in one system | |||||
Non-food | |||||
 | Zea mays | Approximately 0.45% of dry weight [159] | Cheap protein production | Transport of genetic information via pollen (if not transplastomic) | Possible glycosylation effects |
Simultaneous biomass and enzyme production | Long transformation procedure | Subcellular targeting inside the plant cell very important for expression efficiency | |||
System already used for biofuel production | Food |