A whole conglomeration of biochemistry, molecular biology, microbiology,
biochemical and metabolic engineering – biotechnology – has led to the development of routes to a lot of speciality chemicals, starting from cheap carbon sources
(usually carbohydrates, such as corn-steep liquor, molasses
11 or starch) and cocktails of salts, by using living (viable) whole cells. Such syntheses proceed through a
multitude of steps and require fermentation, since they constitute de novo biosynthesis. Products from traditional fermentation are usually obtained with very high
product titers because they are derived from the primary metabolism and thus have
low toxicity. Typical products are organic acids (e.g. acetic, lactic, citric, succinic,
itaconic, pyruvic acid), α-amino acids (e.g. L-Glu, L-Lys, L-Arg, L-Asp) and alcohols (e.g. 1-butanol, 1,3-propanediol).
Secondary metabolites, such as steroids (e.g. hydrocortisone), terpenoids
(e.g. taxadiene, artemisinic acid) and antibiotics (e.g. penicillins, cephalosporins,
tetracyclins) require more sophisticated pathway engineering and are obtained with
reduced productivities due to their inherent toxicity [103–106]. Although many
natural antibiotics were chemically synthesized, these processes are hopelessly
uneconomic compared to the biotechnological routes. However, to date, many
active pharmaceutical ingredients are derivatives of naturally occurring antibiotics,
whose biologically synthesized backbone was chemically modified to reduce their
toxicity and enhance their bioactivity and stability.
In contrast, the majority of microbially mediated biotransformations makes
use of only a single (or a few) biochemical synthetic step(s) by using
(or rather ‘abusing’!) the microbe’s enzymatic potential to convert a nonnatural
organic compound into a desired product. Here, non-growing ‘resting’ cells are
employed. Due to their constrained metabolism, less side reactions occur. The
characteristics of processes using resting versus fermenting whole cells are outlined
in Table 1.3.
Facilitated by rapid advances in molecular biology, the use of wild-type
microorganisms from natural environments possessing >4000 genes
12 (which
often show decreased yields and/or stereoselectivities due to competing enzyme
activities) is constantly declining, while the application of recombinant cells
(over)expressing the required protein(s) is rapidly increasing. Consequently, the
catalytic protein becomes the dominant fraction in the cell’s proteome and side
reactions become negligible. If required, competing enzymes can be knocked out
completely, as long as they are not of vital importance for the primary metabolism.
Such taylor-made genetically engineered organisms for biotransformations are
often called ‘designer bugs’.
11 Cheap forms of sucrose.
12 E. coli has ~4500 genes and Saccharomyces cerevisiae (baker’s yeast) ~6500 genes.
1.3 Advantages and Disadvantages of Biocatalysts
9
biochemical and metabolic engineering – biotechnology – has led to the development of routes to a lot of speciality chemicals, starting from cheap carbon sources
(usually carbohydrates, such as corn-steep liquor, molasses
11 or starch) and cocktails of salts, by using living (viable) whole cells. Such syntheses proceed through a
multitude of steps and require fermentation, since they constitute de novo biosynthesis. Products from traditional fermentation are usually obtained with very high
product titers because they are derived from the primary metabolism and thus have
low toxicity. Typical products are organic acids (e.g. acetic, lactic, citric, succinic,
itaconic, pyruvic acid), α-amino acids (e.g. L-Glu, L-Lys, L-Arg, L-Asp) and alcohols (e.g. 1-butanol, 1,3-propanediol).
Secondary metabolites, such as steroids (e.g. hydrocortisone), terpenoids
(e.g. taxadiene, artemisinic acid) and antibiotics (e.g. penicillins, cephalosporins,
tetracyclins) require more sophisticated pathway engineering and are obtained with
reduced productivities due to their inherent toxicity [103–106]. Although many
natural antibiotics were chemically synthesized, these processes are hopelessly
uneconomic compared to the biotechnological routes. However, to date, many
active pharmaceutical ingredients are derivatives of naturally occurring antibiotics,
whose biologically synthesized backbone was chemically modified to reduce their
toxicity and enhance their bioactivity and stability.
In contrast, the majority of microbially mediated biotransformations makes
use of only a single (or a few) biochemical synthetic step(s) by using
(or rather ‘abusing’!) the microbe’s enzymatic potential to convert a nonnatural
organic compound into a desired product. Here, non-growing ‘resting’ cells are
employed. Due to their constrained metabolism, less side reactions occur. The
characteristics of processes using resting versus fermenting whole cells are outlined
in Table 1.3.
Facilitated by rapid advances in molecular biology, the use of wild-type
microorganisms from natural environments possessing >4000 genes
12 (which
often show decreased yields and/or stereoselectivities due to competing enzyme
activities) is constantly declining, while the application of recombinant cells
(over)expressing the required protein(s) is rapidly increasing. Consequently, the
catalytic protein becomes the dominant fraction in the cell’s proteome and side
reactions become negligible. If required, competing enzymes can be knocked out
completely, as long as they are not of vital importance for the primary metabolism.
Such taylor-made genetically engineered organisms for biotransformations are
often called ‘designer bugs’.
11 Cheap forms of sucrose.
12 E. coli has ~4500 genes and Saccharomyces cerevisiae (baker’s yeast) ~6500 genes.
1.3 Advantages and Disadvantages of Biocatalysts
9