Microbial Metabolism

Metabolism – chemical reactions in cells required by organisms to assimilate energy and use it to grow and reproduce.

Types of metabolic reactions based on rearrangement of molecules and energy transfer processes

Catabolic Reactions

• (degradative)
• Energy is released from the break down of complex organic compounds into simpler substances.

Anabolic reactions

• (biosynthetic)
• energy is required to combine simpler substances to form more molecules.

Note: Energy released from catabolic reactions is stored by ATP for use in anabolic reactions (i.e., concept of coupled chemical reactions in cells)

Enzymes – proteins produced by cells to catalyze chemical reactions by “lowering the activation energy”

When enzymes and substrates combine, substrate is transformed, enzyme is recovered.

Most enzymes consist of a protein portion and a cofactor (can be a metal or a complex organic, coenzyme).

Enzyme specificity to its substrate is a function of the 3D shape of the enzyme’s active site.

Names end with – ase; based on substrate’s name, e.g., urease & lipase; based on type of reaction, e.g., oxidase & hydrolase

 4 Types of enzymes:

Exoenzymes (extracellular) vs. endoenzymes (intracellular)

Constitutive (constant amounts) or inducible (only produced when substrates are present) enzymes.

Sensitivity of enzyme activity to environment

• enzymes function optimally under conditions in an organism’s natural habitat (a direct consequence of natural selection); changes in these normal conditions may denature them, preventing substrates from attaching to their active sites; e.g., high temperature, extreme pH, presence of certain chemicals as alcohol or heavy metals

Regulation of enzyme activity

• competitive inhibition of enzyme activity results from the presence of other molecules that have similar structure as the substrate, e.g., sulfa drugs mimicking PABA competing for enzyme’s active site
• feedback inhibition* – product of a reaction inhibits enzyme activity in the pathway, e.g., allosteric enzymes with active site and regulatory site
• repression vs. induction of enzyme synthesis by regulating the way genes are “expressed”

Cellular respiration

Carbohydrate catabolism –

• converting chemical energy in carbohydrate molecules into available chemical energy in ATP

• Glycolysis (EMP)– oxidation of glucose (6C) to produce pyruvate (3C); ATP & NADH (ecarrier) are released

• Aerobic respiration: O2 as the ultimate oxidizing agent, triggers the “complete” oxidation of glucose (fuel molecule) into water & CO2.

• Krebs Cycle (TCA) – further oxidation of fuel molecules that yield ATP, NADH, FADH2 & CO2

• Electron transport chain – electrons transported through a series of carriers in membrane, generate energy used by ADP & phosphate to produce ATP (oxidative phosphorylation

Other examples of fermentation in bacteria & their products

Acetic acid fermentation produces acetic acid (vinegar)

Mixed acid fermentation produces a complex & variable mixture of acids

Butanediol fermentation produces mostly butanediol (glycol, toxic substance in antifreeze)

Lipid catabolism – lipases hydrolyze fats into glycerol & fatty acids, oxidation of fatty acids yields products that enter part of glycolysis or Krebs cycle

Protein catabolism – amino acids are converted to products that enter the Krebs cycle

Anabolism (Biosynthesis) & amphibolism – most catabolic pathways contain strategic molecular intermediates that can be diverted into anabolic pathways; this property is called amphibolism.

Photosynthesis – conversion of light energy into chemical energy (stored in carbohydrates) through carbon fixation; biosynthesis of carbohydrates