Cells

Cells are a different class of biocatalyst in that they contain considerable arrays of molecular biocatalysts (such as enzymes), which they harness in sequential step reactions to catalyse extensive biotransformations (e.g.glucose-CO2+H2O). Therefore, the range and scale of biocatalysis within a particular cell is vastly greater than that of particular extracellular enzymes, ribozymes, etc. Cell-based biocatalysis can be achieved with animal, plant and microbial sources. Cell-based systems are particularly valuable for multiple-step reactions and reactions that involve complex energy transduction such as ATP hydrolysis.At first sight it may seem that the need for extracellular enzymes is questionable.
However, cells are living organisms and their priority is life support, rather than completion of a biotransformation required by a biotechnologist. Consequently, biochemical resources and energy are used by cells for growth and so cells are not efficient biocatalysts for many simple biotransformations.Sometimes cells need to be ‘tricked’ into switching on a metabolic pathway, which is dormant because the cells have no need for it, when the biotechnologist has a need for the pathway to complete a biotransformation. Cells are important biocatalysts for complex and multistep reactions and the merits of various cells as biocatalysts are discussed briefly below.

1. Animal Cells
Commercial applications for animal cells are increasing and they are now routinely used for expression of proteins from recombinant DNA. They are also used as hosts for attenuated strains of important viruses in the production of vaccines for foot-and-mouth disease, polio, rabies, measles and rubella, etc.
A particularly important catalytic biotransformation of animal cells is post-translational modification of commercial proteins. Many key proteins receive biochemical modification after biosynthesis (translation of mRNA to protein) and such modifications are essential for proper functioning of a mature protein.Bacterial cells can be used to express commercial human proteins, but bacteria cannot complete the posttranslational modifications needed to make a protein fully functional.
Post-translational modifications include glycosylation (adding sugar residues), formation of disulfide bridges, amidation, carboxylation or phosphorylation of amino acid residues and highly specific proteasebased cutting of a protein chain to produce a particular protein shape.
However, animal cells are fragile, have special growth requirements, have low product yields and are susceptible to infection by bacteria and viruses. Animal cell culture is therefore comparatively expensive and normally reserved for high-value medical products.
2. Plant Cells
In addition to crops, many plants produce compounds that have commercial value. Over three-quarters of the 32000+ known natural products are derived from plants.These include medicines and drugs such as atropine, morphine and digoxin, essential oils and fragrances such as menthol, strawberry, vanilla and camphor, pigments such as anthocyanin, betacyanin and saffron, and speciality products such as enzymes, fungicides, pesticides, peptides, vitamins and pigments.Perhaps the most notorious plant products are narcotics such as opium, cocaine and morphine.
Greater development of plant biocatalysis has been slow due to the disadvantages associated with whole plant cultivation, such as weather requirements, low product yield, geographical complications, pesticide/herbicide requirements and expensive extraction processes. A particular problem relates to plant vacuoles that accumulate waste products.
During cell disruption to release plant protein products, waste materials are also released that contaminate the product and often cause disruption/inactivation of the final protein product.
Advances in plant genetic engineering will enable a whole range of products, which were previously difficult to obtain from plants, to be produced in quantity from domesticated crop plants. Transgenic plants are beginning to compete with microbial cell systems for bulk production of biomolecules. Other improvements in plant cell tissue culture have permitted processes that provide a suitable alternative to whole plant cultivation for speciality low-volume products.
3. Microorganisms (Bacteria, Yeast and Filamentous Fungi)
Microorganisms are without question the most versatile and adaptable forms of life and this ability has enabled them to survive on this planet for over 3 billion years. A key feature in this remarkable success is an immense capacity that microorganisms have for biocatalysis. We have
made extensive use of the biocatalytic properties of microorganisms in production of beverages and foodstuffs, and this is a common factor shared by all communities in the world.
Advances in microbiological sciences over the past 70 years have revealed an enormous biocatalytic potential inherent in microorganisms and this has stimulated new contributions to medicine, agriculture, waste (water and hazardous) management and animal feed. It is clear that microorganisms have considerably more potential for biotransformation of a wide range of organic and biochemicals and current opinion suggests that less than 3% of the total range of microorganisms on Earth have been thoroughly characterised in terms of biocatalytic potential. As biocatalysts, some microorganisms are producers of organic material (autotrophs), whereas some are consumers of organic material (heterotrophs), and although bacteria are prokaryotic (i.e. they lack nuclear membrane, mitochondria, endoplasmic reticulum, Golgi apparatus and lysosomes) they possess sophisticated and flexible catalytic systems for both biosynthesis and biodegradation. Although easier to cultivate than animal or plant cells, they are not without some drawbacks as biocatalysts. A critical concern is safety and assurance that products are free from residual bacterial/fungal toxins/cells.