Table 1 Advantages and disadvantages of homologous and heterologous protein expression in different host organisms in regard to technical, ethical and economic aspects

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)

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

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

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

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