CHAPTER 5

MICROBIAL METABOLISM

• Energy – Chemical work– Transport work– Mechanical work

• Laws of thermodynamics– 1st

– 2nd – entropy

• Two fundamental tasks required for growth and reproduction– Catabolism

– Anabolism

• Metabolism – Amazing diversity but also unity

• Ordered, enzyme-mediated pathways• ATP• Redox

• Catabolic reactions– hydrolytic, exergonic (-ΔG)– Keq>1, spontaneous – Cellular respiration

• Provides precursor molecules and energy for anabolic reactions

• Anabolic reactions– dehydration synthesis, endergonic (+ΔG)– Keq<1, not favorable – protein synthesis

• Consumes energy and precursor molecules in the biosynthesis of macromolecules

Amphibolic reactions

Energy of Activation (EA)

• Catalysts (influence reaction rate):– Temperature

– Substrate Concentration

– Enzymes – biological catalysts

• Enzymes:• All protein or holoenzymes

– Apoenzyme + Cofactor (coenzyme)

• Characteristics of enzymes– do not make reactions happen that could not

happen on their own– not permanently altered or used up– substrate-specific– Function is based on structure

• Six functional categories of enzymes:

Unconventional Enzymes

• Ribozymes– Novel type of RNA

• Extremozymes– Have molecular applications

Mechanism of Enzymatic Action

Induced Fit model

Factors influencing enzyme activity

• Denaturing stresses – Heat, pH, UV radiation, chemicals

• Substrate concentration• Competitive inhibition• Non-competitive (allosteric) inhibition

Feedback Inhibition

•Negative allosteric effection

Reduction-Oxidation Reactions

• Redox reactions liberate energy

– always coupled – oxidation (electron donor)

– reduction (electron acceptor)

• Standard reduction potential (E`O)

• Reducing power (potential energy)

Reduction of NAD – common electron carrier

•Electrons and protons are typically removed together•The equivalent of a hydrogen atom

NAD and FAD are common electron carriers

E’0 of various biologically important redox couples

Electrons moving toward less negative acceptors release free energy

Amount of energy released correlates with magnitude of difference in E’0

ATP Synthesis

• Free energy used to phosphorylate ADP forms ATP

–metabolic money!

• Substrate level phosphorylation– chemical energy

• Oxidative phosphorylation – energy from proton motive force

• Photophosphorylation– radiant energy

Heterotrophic Metabolism

• Oxidize energy (electron) rich organic molecules

• Typically utilize carbohydrates – Glucose (C6H12O6) is #1 source

• Three possible pathways based on final electron receptor

–Aerobic respiration – exogenous (oxygen)–Anaerobic respiration – exogenous –Fermentation – endogenous organic

pathways are amphibolic and provide

• Energy• Reducing power• Precursor metabolites

– Respiration uses reducing power to generate ATP• NADH and FADH2 provide electrons to power proton

motive force

• Terminal electron acceptor varies – Oxygen in aerobic respiration– Anaerobic respiration uses alternate inorganic

molecule

GLYCOLYSIS

• Embden-Meyerhof Pathway

• Common pathway

• Glucose (6C) partially broken down into 2 molecules of pyruvate (3C)– Anerobic; cytoplasm

• 2 NADH; 4 ATP

• 2 ATP consumed; so net gain of 2 ATP – Substrate level phosphorylation

• Pentose phosphate pathway– Produces many intermediate materials for

other pathways• glyceraldehyde 3-phosphate, fructose 6-

phosphate, ribulose 5-phosphate• If only 5 carbon sugars are available it can

biosynthesize 6 carbon sugars

– Major contributor to biosynthesis• reducing power in NADPH• vital precursor metabolites for anabolic pathways• intermediates may be used to generate ATP

Pentose phosphate pathway

• Entner-Doudoroff pathway– Alternate pathway to glycolysis – typically not seen in G+ bacteria – major contributor to biosynthesis

• reducing power as NADH and NADPH• vital precursor metabolites for anabolic pathways

Entner-Doudoroff pathway