Several pathways exist in cells from substrate 2H to oxygen, namely, direct oxidation, the direct cytochrome pathway, and the indirect cytochrome pathway.
When the dissolved oxygen reaches the microbial cell surface through the bulk liquid pathway, the molecular oxygen may be transported to the interior of the cell through cell wall and cell membrane by mechanisms like diffusion, active transport, and others.
A typical active transport mechanism scheme of oxygen transfer inside the cell has been depicted in literature.
After getting inside the cells, oxygen may participate in intercellular reaction in several pathways as depicted mechanistically in Fig. 5.23. Many microorganisms are provided with a mechanism whereby the hydrogen removed by dehydrogenation may be passed to gaseous oxygen. In this process (called cell respiration), oxygen gas serves as the terminal electron acceptor.
This mechanism is also shown in the literature. The available electrons in these reactions may yield energy in a form available to the cell. In bacterial physiology in this pathway to oxygen, the removal of low hydrogen atom does not generally take place by reacting with molecular oxygen without the participation of enzymes and coenzymes. Several pathways exist in cells from substrate 2H to oxygen, namely, direct oxidation, the direct cytochrome pathway, and the indirect cytochrome pathway.
In certain oxidase enzymes, for example, the d- or l-amino acid oxidases, the involved enzyme construction can directly react with oxygen. In this pathway (Fig. 5.23), these enzymes are known to be capable of combining the 2H from the substrates. This pathway may be vital for generation of intermediates that are required by the cell.
In some organisms, a few dehydrogenases (e.g., succinic dehydrogenase, Fig. 5.23 A) are capable of directly reducing a series of cytochrome pigments. The latter are capable of reacting with oxygen to form water. This pathway comprises the only path out to oxygen, but it is generally of much smaller significance. An example of a cytochrome pathway is shown in Fig. 5.23B.
This oxygen pathway may yield energy to the cell. The indirect cytochrome pathway to oxygen as shown in Fig. 5.23C is probably the most important. In many microorganisms, dehydrogenases reduce the nicotinamide coenzymes. In the reduced state, these coenzymes neither react with molecular oxygen nor reduce the cytochrome pigments, but they are able to reduce certain flavo-proteins. Many such flavo-proteins can react with the cytochrome pigments producing oxygen. Oxidation along this pathway (as shown in Fig. 5.23C) may yield energy in a form available to the cell.