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 function 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 passing through the cell wall. In medicine, many exoenzymes (hyaluronidase, collagenase) are known as pathogenicity factors — "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. Next, bacteria use one of three pathways to convert the substrate into pyruvate:
- Glycolytic pathway (Embden–Meyerhof–Parnas): the classical cleavage variant via fructose-1,6-diphosphate.
- Pentose phosphate pathway: necessary for preparing metabolites for biosynthesis (amino acids, nucleic acids) and synthesizing reduced NADP. It is rarely the sole pathway, but occurs in bacteria lacking glycolytic enzymes (e.g., Lactobacillus brevis).
- Entner–Doudoroff pathway: proceeds via the formation of 2-keto-3-deoxy-6-phosphogluconic acid. 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-coenzyme A, which enters the tricarboxylic acid cycle (TCA cycle). This cycle is amphibolic (performs a dual function):
- Catabolic: complete oxidation of substances to carbon dioxide with hydrogen removal to carriers (NAD).
- Anabolic: intermediate products (oxaloacetate, alpha-ketoglutarate) are used for the synthesis of amino acids, fats, and pyrimidine nucleotides.
In bacteria with oxidative metabolism, electrons are sent to the respiratory chain on the cytoplasmic membrane. The chain involves 4 classes of carriers: NAD, flavoproteins, quinones (naphthoquinones may replace ubiquinones), and cytochromes.
The outward transport of protons creates an electrochemical gradient, the energy of which is spent on ATP synthesis (chemiosmotic coupling). The final acceptor is molecular oxygen, to which electrons are transferred by cytochrome c oxidase (cytochrome complex $a + a_3$) via a change in iron valence. If cytochromes are absent, flavoproteins perform 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 proceeds 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. The amino acid is degraded with the release of ammonia and the formation of a keto acid, which enters the TCA cycle. Deep decomposition 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.
Briefly: answers to topical questions
What are the 6 classes of enzymes in the classification?
The classification based on the type of catalyzed reaction distinguishes 6 standard classes of enzymes.
- Oxidoreductases — catalyze oxidation-reduction reactions (electron transfer).
- Transferases — catalyze the transfer of functional groups from one substance to another.
- Hydrolases — catalyze hydrolysis (cleavage of complex organic substances into simple ones by the addition of water).
- Lyases — catalyze non-hydrolytic addition or removal of functional groups (bond cleavage by non-hydrolytic means).
- Isomerases — catalyze isomerization reactions (movement of groups within a single molecule).
- Ligases (synthetases) — catalyze synthesis (ligation) reactions using ATP energy.
Class and exam questions
Where is the respiratory chain localized in bacteria?
The process occurs on the cytoplasmic membrane and intracellular membrane structures (its derivatives), as bacteria lack mitochondria.
What is the difference between oxidative metabolism and fermentation?
In oxidative metabolism (respiration), the substrate is completely oxidized in the TCA cycle to carbon dioxide and water with a high ATP yield. In fermentation, oxidation is incomplete, the energy yield is lower, and the end products are acids, alcohols, and gases.
Why do bacteria need exoenzymes?
They hydrolyze large macromolecules in the environment into monomers and dimers that can pass through the cell wall. In pathogens, they also act as aggressive factors, destroying host tissues.
What else to study on this topic
- Features of the Entner–Doudoroff pathway in non-fermenting bacteria
- The link between TCA cycle intermediates and pyrimidine nucleotide synthesis
- The chemical mechanism of cytochrome c oxidase and electron transfer to oxygen
- Alternative respiration pathways without the involvement of cytochromes
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