Bacterial Metabolism

Bacterial metabolism definition

Metabolism is referred to as the sum of all chemical reactions (anabolic + catabolic) happening in an organism. These series of biochemical reactions are essential to sustain life. Thus, bacterial metabolism is the sum of all chemical reactions happening in bacteria.

We can classify bacterial metabolism based on what a bacterium uses as a source of energy:

• If the bacteria use light as an energy source, they are called phototrophs.
• If the bacteria use the oxidation of organic/inorganic compounds as an energy source, we call them chemotrophs.

Now, when it comes to a carbon source, bacteria can be an autotroph or a heterotroph. Autotrophs use CO₂ as their main carbon source to create their own organic matter, whereas heterotrophs use organic molecules from other organisms as a carbon source.

Types of bacterial metabolism

Bacteria have different types of metabolism, such as aerobic cellular respiration, fermentation and anaerobic respiration, depending on the availability and their resistance to oxygen.

Bacterial energy metabolism

Metabolism is any chemical reaction that happens in a cell, so bacterial metabolism is the sum of chemical reactions within a bacteria cell. These chemical reactions can be divided into the reactions that produce energy and the ones that are used for other purposes, such as creating organic molecules or increasing the availability of certain elements like nitrogen. In this section we will focus on the first category.

Aerobic cellular respiration

As its name suggests, aerobic cellular respiration is defined as the process of breaking down glucose to make energy in the presence of oxygen. Glucose is a monosaccharide.

Fig. 2. Closed structure (Hawthorn projection) of alpha and beta glucose. The difference between the isomers is the position of the hydroxyl group attached to carbon 1. When several alpha-glucose molecules are joined, starch is generated. When several beta-glucose molecules are joined, cellulose is generated. Humans can digest starch, but not cellulose because the enzymes needed for each reaction are different.

In prokaryotic organisms, most stages of cellular respiration happen in the cytoplasm. The overall chemical formula for aerobic cellular respiration is shown below.

$$ C_{6}H_{12}O_{6} (glucose)+6O_{2} \longrightarrow 6CO_{2} + 6H_{2}O +energy (ATP + heat) $$

Aerobic cellular respiration in prokaryotes can be divided into four phases:

1. Glycolysis

2. Pyruvate oxidation

3. Citric acid cycle (Krebs cycle)

4. Electron transport chain and chemiosmosis (happens in the plasma membrane)

During aerobic cellular respiration, there are two ways in which ATP can be made: substrate-level phosphorylation and oxidative phosphorylation. In substrate-level phosphorylation, an enzyme and a substrate are used to transfer a phosphate group to ADP, creating small amounts of ATP in the stages of glycolysis and the citric acid cycle.

Oxidative phosphorylation, on the other hand, produces high amounts of ATP in the stages of the electron transport chain and chemiosmosis. Oxidative phosphorylation makes use of energy from redox reactions in the electron transport chain to phosphorylate ADP.

Glycolysis

Glycolysis is a type of carbohydrate metabolism that occurs in the cytoplasm of cells. It is the first step of cellular respiration. Glycolysis consists of many enzymatic reactions that break down glucose into two pyruvate molecules. Energy is also produced in the form of ATP, as glucose gets processed. The process of glycolysis does not require oxygen (O)!

$$ \text{Glucose} + 2\text{ }NAD^{+}+\text{ }2\text{ }ADP +\text{ }2\text{ }Pi \longrightarrow 2 \text{ pyruvate} + \text{2 }NADH\text{ }+\text{ }2\text{ }ATP $$

Pyruvate oxidation

After glycolysis, the pyruvate molecules are produced to enter the mitochondria and react with an enzyme called pyruvate dehydrogenase. This enzyme takes out carbon and two oxygen molecules from the pyruvate and adds coenzyme A, producing acetyl-CoA, NADH and CO₂. Acetyl-CoA can also come from fatty acids and amino acids. This phase is called pyruvate oxidation.

Citric acid cycle (Krebs Cycle)

After pyruvate oxidation, we have the citric acid cycle. The citric acid cycle happens in the cytoplasm for prokaryotes, and in the mitochondria matrix for eukaryotes. Acetyl-CoA is the first intermediate of the citric acid cycle. After a molecule of glucose undergoes glycolysis, pyruvate oxidation, and the citric acid cycle, we are left with 10 molecules of NADH, 4 molecules of ATP, 2 molecules of FADH₂ and 6 molecules of CO₂.

Electron transport chain and chemiosmosis

The fourth stage of aerobic cellular respiration consists of the electron transport chain and chemiosmosis. The electron transport chain consists of proteins embedded in the plasma membrane of prokaryotic cells.

In this stage, the prokaryotic electron transport chain gets energy from the electrons donated by NADH and FADH₂. When NADH donates an electron, it becomes NAD+. Similarly, when FADH2 donates an electron, it turns into FAD. Then, the electron transport chain makes use of this energy caused by the movement of electrons to pump protons (H⁺) from the mitochondrial matrix, across the inner mitochondrial membrane, and into the intermembrane space of the mitochondria.

The electrons will keep through the electron transport chain until they reach the final electron acceptor, which is oxygen gas (O₂). This oxygen molecule reacts with the H⁺ to form water (H₂O).

After the electron transport chain builds the proton (H⁺) gradient, chemiosmosis is used to capture their energy. Chemiosmosis is referred to as the diffusion of the H⁺ ions across a membrane from an area of high concentration to an area of low concentration.

Anaerobic cellular respiration

Bacteria that have the ability to survive and make ATP without needing oxygen use anaerobic respiration. In anaerobic cellular respiration, glycolysis happens normally. Then, the pyruvate molecules produced by glycolysis can undergo fermentation.

Lactic acid fermentation is a pathway that uses NADH to reduce pyruvate into lactic acid and NAD⁺.

In alcohol fermentation, pyruvate gets reduced by NADH to form ethanol and NAD⁺.

Nitrogen metabolism

Some bacteria are capable of nitrogen fixation. These nitrogen-fixing bacteria are vital to the nitrogen cycle because they are responsible for converting atmospheric nitrogen (N₂) into ammonia (NH₃).

Bacterial metabolic pathways

Bacterial metabolic pathways consist of a series of enzymatic reactions that cause the alteration of a substrate multiple times before arriving at the final product. Bacteria can have many metabolic pathways, such as the ones we saw above, and many others, such as photosynthesis, lipid metabolism, amino acid metabolism, and nucleotide metabolism.

To figure out the metabolic pathways present in bacteria, scientists use genome annotation. Some specific pathways found in this genome include glycolysis, citric acid cycle, fatty acid metabolism, oxidative phosphorylation, photosynthesis, purine metabolism, methane metabolism, nitrogen metabolism, sulfur metabolism, and even caffeine metabolism!

Iron-reducing bacteria metabolism

Lastly, let's talk about the metabolism of iron-reducing bacteria. These bacteria convert ferric ions (Fe³⁺) into ferrous ions (Fe²⁺).

Bacterial metabolism can be a little overwhelming. But, with time and patience, you will be able to tackle it!