Microbial Metabolism: Foundations and Pathways CH-5

Microbial Metabolism

Introduction to Metabolism

Metabolism encompasses all controlled biochemical reactions that occur within a microbe. The ultimate function of metabolism is to enable the reproduction of the organism by managing the acquisition and use of energy and nutrients.

• Metabolism: The sum of all chemical reactions in a cell.

• Metabolic processes are guided by fundamental principles, including nutrient acquisition, energy extraction, and the synthesis of macromolecules.

Catabolism and Anabolism

Metabolism is divided into two major classes of reactions: catabolic and anabolic pathways.

• Catabolic pathways: Break down larger molecules into smaller products; these reactions are exergonic (release energy).

• Anabolic pathways: Synthesize large molecules from the smaller products of catabolism; these reactions are endergonic (require energy input).

Basic Chemical Reactions Underlying Metabolism

Oxidation and Reduction Reactions

Oxidation-reduction (redox) reactions involve the transfer of electrons from an electron donor to an electron acceptor. These reactions always occur simultaneously and are essential for energy extraction in cells.

• Cells use electron carriers such as NAD+, NADP+, and FAD to shuttle electrons.

ATP Production and Energy Storage

Organisms release energy from nutrients, which is stored in high-energy phosphate bonds of ATP. Cells phosphorylate ADP to ATP in three ways:

• Substrate-level phosphorylation

• Oxidative phosphorylation

• Photophosphorylation

Anabolic pathways use the energy of ATP by breaking a phosphate bond.

The Roles of Enzymes in Metabolism

Enzymes are organic catalysts that increase the likelihood of a reaction by lowering the activation energy required.

• Enzymes are classified into six categories based on their mode of action: hydrolases, isomerases, ligases/polymerases, lyases, oxidoreductases, and transferases.

Enzymes may require nonprotein cofactors (inorganic ions or organic coenzymes) to be active. The combination of an apoenzyme and its cofactor forms a holoenzyme.

Enzyme Classification Table

Class	Type of Reaction Catalyzed	Example
Hydrolase	Hydrolysis (catabolic)	Lipase—breaks down lipid molecules
Isomerase	Rearrangement of atoms within a molecule	Phosphoglucoisomerase—converts glucose 6-phosphate into fructose 6-phosphate
Ligase/Polymerase	Joining two or more chemicals together (anabolic)	Acetyl-CoA synthetase—forms acetyl-CoA
Lyase	Splitting a chemical into smaller parts without water (catabolic)	Fructose-1,6-bisphosphate aldolase
Oxidoreductase	Transfer of electrons or hydrogen atoms	Lactic acid dehydrogenase
Transferase	Moving a functional group from one molecule to another	Hexokinase—transfers phosphate from ATP to glucose

Enzyme Activity and Regulation

Enzyme activity is influenced by several factors:

• Temperature

• pH

• Enzyme and substrate concentrations

• Presence of inhibitors (competitive and noncompetitive)

Enzyme activity can be regulated by activators (allosteric activation) or inhibitors (competitive and noncompetitive). Feedback inhibition is a common regulatory mechanism in metabolic pathways.

Carbohydrate Catabolism

Overview of Glucose Catabolism

Many organisms oxidize carbohydrates, primarily glucose, as their main energy source. Glucose catabolism occurs via two main processes: cellular respiration and fermentation.

Glycolysis

Glycolysis is the metabolic pathway that splits a six-carbon glucose into two three-carbon pyruvic acid molecules. It occurs in the cytoplasm and involves substrate-level phosphorylation.

• Net gain: 2 ATP, 2 NADH, and 2 pyruvic acid molecules per glucose.

• Divided into three stages: energy-investment, lysis, and energy-conserving.

Cellular Respiration

Cellular respiration completely oxidizes pyruvic acid to produce ATP through a series of redox reactions. It consists of three stages:

1. Synthesis of acetyl-CoA

2. Krebs cycle (citric acid cycle)

3. Electron transport chain (ETC)

Summary Table: Ideal Prokaryotic Aerobic Respiration of One Glucose

Pathway	ATP Produced	ATP Used	NADH Produced	FADH2 Produced
Glycolysis	4	2	2	0
Synthesis of acetyl-CoA & Citric acid cycle	2	0	8	2
Electron transport chain	34	0	0	0
Total	40	2
Net Total	38

Electron Transport Chain (ETC) and Chemiosmosis

The ETC is a series of carrier molecules that transfer electrons to a final electron acceptor, generating a proton gradient used to produce ATP via chemiosmosis. In aerobic respiration, oxygen is the final electron acceptor; in anaerobic respiration, other molecules serve this role.

Fermentation

Fermentation provides an alternative source of NAD+ when cells cannot completely oxidize glucose by cellular respiration. It involves the partial oxidation of sugar using an organic molecule as the final electron acceptor.

Comparison Table: Aerobic Respiration, Anaerobic Respiration, and Fermentation

	Aerobic Respiration	Anaerobic Respiration	Fermentation
Oxygen Required	Yes	No	No
Type of Phosphorylation	Substrate-level & oxidative	Substrate-level & oxidative	Substrate-level
Final Electron Acceptor	Oxygen	NO3-, SO42-, CO32-, or other	Cellular organic molecules
ATP Yield (per glucose)	38 (prokaryotes)	4–36	2

Other Catabolic Pathways

Lipid and Protein Catabolism

Lipids and proteins can be catabolized to provide energy and precursor metabolites for cellular processes. Lipids are broken down into glycerol and fatty acids, while proteins are degraded into amino acids, which are then deaminated.

Photosynthesis

Overview and Structures

Photosynthesis is the process by which organisms synthesize organic molecules from CO2 and H2O using light energy. Chlorophylls and photosystems are essential components, with light-dependent and light-independent reactions working together to produce carbohydrates.

Other Anabolic Pathways

Anabolic Reactions and Precursor Metabolites

Anabolic reactions synthesize complex molecules from simpler ones, requiring energy and precursor metabolites. Many anabolic pathways are the reverse of catabolic pathways and are termed amphibolic if they can proceed in either direction.

Integration and Regulation of Metabolic Function

Regulation Mechanisms

Cells regulate metabolism by controlling enzyme synthesis and activity, using mechanisms such as feedback inhibition, allosteric regulation, and compartmentalization of metabolic pathways in eukaryotes.

Additional info: This guide covers the foundational concepts of microbial metabolism, including the major catabolic and anabolic pathways, enzyme function and regulation, and the integration of metabolic processes. It is suitable for exam preparation in a college-level microbiology course.