Introduction to Microbial Metabolism and Physiology

Exam Tip: Remember the relationship between energy and bonds. Breaking bonds (Catabolism) releases energy (Exergonic), while forming bonds (Anabolism) requires energy (Endergonic).

Biological Catalysts: Enzymes

Chemical reactions in a cell would occur too slowly to sustain life without help. This is where enzymes come in. Enzymes are biologic catalysts that increase the rate of chemical reactions without being consumed in the process or becoming part of the final products.

Enzymes are classified by their location of action:

• Endoenzymes: Produced within a cell and remain inside to catalyze essential intracellular reactions.
• Exoenzymes: Produced within a cell but released to the outside to catalyze extracellular reactions. An example is hyaluronidase, produced by pathogens like Staphylococcus aureus to degrade the "intercellular cement" between human cells, allowing the infection to spread.

The efficiency of an enzyme can be affected by its environment, including optimum pH, temperature, and substrate concentration. Furthermore, certain heavy metals (like lead or mercury) can act as inhibitors, reducing enzyme effectiveness.

Energy Currency: ATP and Redox Reactions

In the complex dance of metabolism, energy must be transferred from catabolic pathways to anabolic ones. This transfer is facilitated by Adenosine Triphosphate (ATP), often called "metabolic money".

ATP is a three-part molecule consisting of a nitrogen base (adenine), a five-carbon sugar (ribose), and a chain of three phosphate groups. The phosphate groups carry negative charges that repel each other; the "strain" on these bonds means that when the terminal phosphate is removed, a significant amount of free energy is released to fuel endergonic reactions.

Energy management also involves redox reactions (oxidation and reduction), which involve the transfer of electrons. Exergonic and endergonic reactions are often "coupled," meaning the energy released from one is immediately used to drive the other.

Pathways of Catabolism

Microbes have evolved several ways to extract energy from nutrients, depending on the availability of oxygen and the types of molecules present.

1. Aerobic Respiration

This is the most efficient way to extract energy, completely catabolizing an organic source (like glucose) into $\text{CO}_2$ using free oxygen as the final electron acceptor. It occurs in three main phases:

1. Glycolysis: Occurs in the cytoplasm; glucose is oxidized into two molecules of pyruvate.
2. Kreb’s Cycle (Citric Acid Cycle): Occurs in the bacterial cytoplasm (or mitochondrial matrix in eukaryotes); it further breaks down pyruvate and releases $\text{CO}_2$.
3. Oxidative Phosphorylation (Electron Transport Chain): Electrons from the previous steps are carried to the ETC. As electrons move through the chain, a proton ($\text{H}^+$) gradient is formed, which drives the enzyme ATP synthase to produce large amounts of ATP.

2. Anaerobic Respiration

Used by organisms that live in environments without oxygen, this process also utilizes glycolysis, the Kreb's cycle, and an ETC. However, instead of oxygen, it uses other oxidized compounds (such as $\text{NO}_3^-$ or $\text{SO}_4^{2-}$) as the final electron acceptor.

3. Fermentation

Fermentation is an incomplete oxidation of glucose. It does not use the Kreb's cycle or an ETC and is much less efficient, producing only about 2 ATP per glucose molecule. It is used to "keep glycolysis going" by regenerating $\text{NAD}^+$ from $\text{NADH}$. Fermentation pathways are named after their end products, such as lactic acid or alcohol.

Clinical Connection: In humans, when oxygen levels are too low (hypoxia), cells switch to anaerobic metabolism. A major byproduct of this is lactic acid. In severe cases like sepsis, high levels of lactic acid can lead to lactic acidosis, which lowers blood pH and indicates poor organ function.