From Cellular Function to Industrial Products
IS
Allosteric regulation of enzymes
Tronslal
mRN)
I &ene transcription
C hrom osom e
Protein-protein interactions
Figure 2.2 Overview of process involved in expressing a specific phenotype in a cell. The chromosome
contains a large number of genes, which represents the genotype of the cell. The genes are transcribed
into mRNA, which is further translated into proteins. Some of these proteins are enzymes involved in
metabolism, e.g., the catabolism of substrates to metabolic products. Some enzymes are allosteric
enzymes, which means that their
in vivo
activity is regulated by other metabolites, e.g., there may be
feed-back inhibition at high concentration of a certain metabolite. Other proteins are involved in overall
regulation of cellular function. Thus, protein kinases may phosphorylate other proteins and hereby
activate or deactivate them. Some proteins are involved in regulation of gene expression, and a feed-back
from the metabolite levels to gene expression via these regulatory proteins is clearly possible.
2.1.2 Transport Processes
Two structures surround the cytoplasm, of most microbial cells, the cell wall and the cytoplasmic
membrane. These structures are normally referred to as the
ceil envelope,
and their chemical
composition is a determining factor for the transport of species between the abiotic phase, i.e., the
exterior of the cell, and the cytoplasm. The cell wall has a rigid structure of cross-linked
dissacharides and peptides (peptidoglycans). Its major function is to prevent the cell from bursting
due to a high intracellular osmotic pressure. The cytoplasmic membrane, which mainly consists of
phospholipids, has a fluid structure with properties that may change dynamically during growth.
Most small molecules can easily pass through the cell wall, and therefore the cytoplasmic
membrane is determining the transport processes. Larger molecules (e.g., proteins) can pass
through the cell wall and the cytoplasmic membrane only if the cell is equipped with special
mechanisms for their excretion. We will not describe these mechanisms further, and in the
following only focus on the transport of small molecules. Furthermore, as the cytoplasmic
membrane is the important barrier between the intracellular and extracellular environment, we
restrict the discussion to transport across this membrane.
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