Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 489

ribosome down one codon toward the C3 end. The energy required for translation of proteins
is significant. The rate of translation varies; it is significantly higher in prokaryotic cells (up to
17 e 21 amino acid residues per second) than in eukaryotic cells (up to 6 e 9 amino acid resi-
dues per second).
When a stop codon is reached, the protein is released from the ribosome with the aid of
a protein release factor (RF). Termination of the polypeptide happens when the A site of the
ribosome faces a stop codon (UAA, UAG, or UGA). No tRNA can recognize or bind to
this codon. Instead, the stop codon induces the binding of a RF protein that prompts the
disassembly of the entire ribosome/mRNA complex. The 70s ribosome then dissociates
into 30s and 50s subunits. An mRNA typically is being read by many (for example, 10 to
20) ribosomes at once; as soon as one ribosome has moved sufficiently far along the message
that the ribosome-binding site is not physically blocked, another ribosome can bind and
initiate synthesis of a new polypeptide chain.
A number of antibiotics act by inhibiting translation; these include anisomycin, cyclohex-
imide, chloramphenicol, tetracycline, streptomycin, erythromycin, and puromycin, among
others. Prokaryotic ribosomes have a different structure from that of eukaryotic ribosomes,
and thus antibiotics can specifically target bacterial infections without any detriment to
a eukaryotic host's cells.
10.4.3. Posttranslational Processing: Making the Product Useful
Often the polypeptide formed from the ribosome must undergo further processing before
it can become truly useful. First, the newly formed polypeptide chain must fold to assume
native secondary and tertiary structures, which is known as protein folding. In some cases,
several different chains must associate to form a particular enzyme or structural protein.
Additionally, chaperones are an important class of proteins that assist in the proper folding
of peptides. There are distinct pathways to assist in folding polypeptides. The level of chap-
erones in a cell increases in response to environmental stresses such as high temperature.
Misfolded proteins are subject to degradation if they remain soluble. Often misfolded
proteins aggregate and form insoluble particles (i.e. inclusion bodies). High levels of expres-
sion of foreign proteins through recombinant DNA technology in E. coli often overwhelm the
processing machinery, resulting in inclusion bodies.
The formation of proteins in inclusion bodies greatly complicates any bioprocess, since in
vitro methods to unfold and refold the protein product must be employed. Even when a cell
properly folds a protein, additional cellular processing steps must occur to make a useful
product. This may include the formation of disulfide bridges or attachment of any of
a number of biochemical functional groups, such as acetate, phosphate, various lipids and
carbohydrates. Enzymes may also remove one or more amino acids from the leading (amino)
end of the polypeptide chain, leaving a protein consisting of two polypeptide chains con-
nected by disulfide bonds.
Many proteins are secreted through a membrane. In many cases, the translocation of the
protein across the membrane is done cotranslationally (during translation), while in some
cases post translation movement across the membrane occurs. When proteins move across
a membrane, they have a signal sequence (about 20 to 25 amino acids). This signal sequence
is clipped off during secretion. Such proteins exist in a preform and mature form. The