Enzymatic Apparatus and Its Significance
Bacteria possess a powerful set of enzymes classified into 6 standard classes (oxidoreductases, transferases, hydrolases, ligases, lyases, isomerases). Their cellular localization strictly determines their function:
- Endoenzymes operate inside the cell, driving intracellular metabolic reactions.
- Exoenzymes are secreted into the external environment. Their main task is the hydrolysis of large macromolecules (polysaccharides, proteins, lipids) into dimers and monomers capable of crossing the cell wall. In medicine, many exoenzymes (hyaluronidase, collagenase) are known as pathogenicity factors or "aggressive enzymes" that destroy host tissues.
- Periplasmic enzymes are located in the periplasmic space and participate in the transport of substances into the cell.
The set of enzymes (enzymatic profile) is strictly determined for each family, genus, and species. By identifying this profile through inoculation on differential diagnostic media, microbiologists identify bacteria.
Carbohydrate Catabolism: From Glucose to Pyruvate
The initial stages of hexose oxidation are universal for both oxidative and fermentative types of metabolism. The starting reaction is the phosphorylation of glucose to glucose-6-phosphate. Bacteria then use one of three pathways to convert the substrate into pyruvate:
- Glycolytic pathway (Embden-Meyerhof-Parnas): the classical pathway of breakdown via fructose-1,6-bisphosphate.
- Pentose phosphate pathway: necessary for preparing metabolites for biosynthesis (amino acids, nucleic acids) and synthesizing reduced NADP. Rarely the sole pathway, but found in bacteria lacking glycolytic enzymes (e.g., Lactobacillus brevis).
- Entner-Doudoroff pathway: proceeds via the formation of 2-keto-3-deoxy-6-phosphogluconate. Characteristic only of bacteria lacking the enzyme phosphofructokinase (e.g., microorganisms of the genus Pseudomonas).
Tricarboxylic Acid Cycle and Respiration
The resulting pyruvate is converted into acetyl-CoA, which enters the tricarboxylic acid (TCA) cycle. This cycle is amphibolic (serves a dual function):
- Catabolic: complete oxidation of substances to carbon dioxide with the release of hydrogen onto carriers (NAD).
- Anabolic: intermediate products (oxaloacetate, alpha-ketoglutarate) are used for the synthesis of amino acids, lipids, and pyrimidine nucleotides.
In bacteria with oxidative metabolism, electrons are transferred to the electron transport chain located on the cytoplasmic membrane. The chain involves 4 classes of carriers: NAD, flavoproteins, quinones (naphthoquinones instead of ubiquinones), and cytochromes.
The outward translocation of protons creates an electrochemical gradient, the energy of which is used to synthesize ATP (chemiosmotic coupling). The terminal acceptor is molecular oxygen, to which electrons are transferred by cytochrome c oxidase (cytochrome complex $a + a_3$) via changes in iron valence. If cytochromes are absent, flavoproteins mediate the transfer, and hydrogen peroxide is formed instead of water.
Proteins as an Energy Source
Bacteria are capable of using proteins for energy. The process occurs in several stages:
- Extracellular proteolytic exoenzymes cleave proteins into peptides.
- Peptides are transported into the cell.
- Intracellular peptidases break them down into amino acids.
The main energy-yielding process for ammonifying bacteria is oxidative deamination. Amino acids are degraded with the release of ammonia and the formation of keto acids, which enter the TCA cycle. Deep breakdown of organic matter (protein mineralization) is called putrefaction. It is accompanied by the formation of foul-smelling primary amines, hydrogen sulfide, and carbon dioxide. Representatives of putrefactive microflora include species of the genera Proteus, Pseudomonas, and Bacillus cereus.