For example, human growth hormone was once available only from the pituitary glands of cadavers. It was very rare, expensive,
and potentially contaminated with prions. In the 1980s, it even transmitted a rare neurological disease (Creutzfeld-Jakob
disease) to patients before anyone was sure what kind of agent caused the disease. Now recombinant bacteria produce human
growth hormone in large quantities that have made it safer and less expensive, and thus available to more patients around
the world.
When those bacteria were genetically modified, the process had only just begun. They had to be cultivated in large enough
numbers to produce therapeutic quantities of the protein. And that meant that it was essential to understand how they grew
and reproduced.
Cell Metabolism
In broad outline, cell metabolism is basically the same in all living things. Oxygen and nutrients are brought inside the
cell, where chemical reactions driven by proteins provide the cell with energy and raw materials. Waste products including
water, carbon dioxide, and ammonia are expelled. These chemical facts of life point to three major concerns in fermentation
and cell culture: nutrients, aeration, and the removal of waste products and heat. There are, however, some significant differences
in the details of how different sorts of cells carry out the process of metabolism.
Bacteria, for instance, are prokaryotes, some of the oldest life forms on Earth. They are simple creatures, basically tiny
capsules of watery cytoplasm in which float DNA and RNA, metabolic enzymes, food, and waste products. Prokaryote metabolism
is relatively simple: They need simple nutrients (like sugar) to burn for energy.
Yeasts and plant and animal cells are a more complex form of life called eukaryotes. In eukaryotes, a nucleus protects the
genetic material, which is stored on chromosomes. Ribosomes outside of the nucleus use RNA to translate that information for
making proteins. Many other inclusion bodies or organelles are found inside the eukaryotic cell: microtubules, vacuoles, mitochondria,
lysosomes, smooth and rough endoplasmic reticuli, ribosomes, and Golgi bodies.
In eukaryotic cells, mitochondria perform much of the metabolic work. They act just like little prokaryotes, employed by the
big cell to do what they're best at: getting energy out of food.
Large nutrient molecules (such as proteins, polysaccharides, and fats) are broken down into smaller, usable pieces (amino
acids, sugars, fatty acid, and glycerol). Those are broken down even further to make, among other things, two very important
molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD). Remaining nutrients are taken up by the
mitochondria, which oxidize them to make more ATP and NAD.
ATP stores energy for all living things, and NAD is important in its production. ATP provides the power needed for most cell
processes: molecular synthesis; transport of materials across the membrane; and cell replication, movement, and maintenance.
Cells for Bioproduction
Today's biotech and biopharmaceutical companies use a variety of bacterial, yeast, animal, and plant cells for production.
The choice of which cell to use depends on a number of factors, some technical and some economic.
Bacteria and yeast, for instance, are relatively simple to grow. Each cell of a bacterium or yeast is an independent organism
capable of its own metabolism. Yeasts and bacteria have fairly simple nutritional needs and grow well suspended in a liquid
medium, even in big fermentors with a capacity of 1,000, 10,000, or more liters. The cell walls of bacteria and yeasts resist
damage, even when the cells are packed closely together and stirred around by mechanical mixers.
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