Overview of Microbial Metabolism

oCells reproduce typically when doubled in size

Enzymes

o   Temperature and pressure inside cell too low and necessitates a catalyst

o   Cofactors

§  Complex organic molecules (coenzmyes) or simple ions

·         Mg++

·         Ca++

·         Fe++

·         Cu++

·         Zn++

§  Many derived from vitamins

·         NAD from niacin (Vit B₁₂)

§  Role

·         Accept atoms or electrons

·         Donate atoms or electrons

·         Electron carriers

·         Ion attraction in binding substrates

o   Factors affecting activity

§  Temperature

§  pH

§  Enzyme and substrate concentrations

§  Inhibitors

Protein portion of enzyme in inactive form is called apoenzyme and nonprotein componet (cofactor) = whole active enzyme (holoenzyme)

Dehydrogenation Reactions

o   Oxidation-reduction reactions where 1 or 2 hydrogen atoms transferred to electron carrier like NAD+

§  Oxidized substrate loses two H atoms, reducing NAD+ to NADH + H+

Bioenergetic Fundamentals

o   Carbohydrates are primary energy source

§  Energy rich

§  Easily broken down

§  Other sources used

o   Chemical degradation of Glucose

§  C₆H₁₂O₆ + 6 O₂ à 6 H₂O + 6CO₂ + 686 kcal/mol

o   Cells oxidize glucose through metabolic pathways and trap and store energy

o   Three basic mechanisms

§  Substrate level phosphorylation

§  Oxidative phosphorylation

§  Photophosphorylation

Substrate Level Phosphorylation

o   Energy trapped in high energy phosphate bond on substrate and then transferred to ATP

o   Example – Phosphoenylpyruvate

o   PEP + ADP à Pyruvate + ATP

  Oxidative Phosphorylation

o   Oxidation energy captured in electrons bound to electron carriers

o   Carriers transfer electrons through electron transport chain à proton gradient formed

o   Electrochemical proton gradient à ATP synthesis (chemiosmosis)

o   Oxygen final electron acceptor à water

Photophosphorylation

o   Energy in light absorbed by exciting chlorophyll electrons

o   Electrons passed through electron transport chain which forms ATP and NADPH

o   Water replaces lost electrons forming oxygen

o   ATP and NADPH à glucose synthesis

Carbohydrate catabolism

o   Glucose most common substrate for energy production

o   Microbes catabolize by

§  Respiration

§  Fermentation

o   Depends on metabolic capabilities and whether electron acceptor like oxygen is present

o   Respiration used if

§  Microbes have ETC and

§  Inorganic electron acceptor present such as O

o   Fermentation used if

§  Inorganic electron acceptor not present or

§  Microbes have no ETC

o   Initial step of both pathways glycolysis (Embden Meyerhof pathway)

§  Glucose à pyruvate

o   Pyruvate’s fate different in fermentation and respiration

Glycolysis

o   10 enzymatic steps to degrade glucose

o   glucose + 2 NAD + 2 ADP + 2 Pi à 2 pyruvate + 2 ATP + 2 NADH

o   Other carbs first converted to glucose, then use glycolysis

o   Alternatives

§  Some microbes degrade glucose by

·         Hexose Monophosphate Shunt

·         Enter-Doudoroff Pathway

Respiration

o   Occurs in 3 stages (eukaryotes happens in mitochondria)

§  Pyruvate à acetyl-CoA

·         5 steps

·         3 different enzymes

·         5 cofactors

o   Thiamine pyrophosphate

o   Lipoic acid

o   CoA

o   FAD

o   NAD

§  Acetyl-CoA à Krebs à CO₂ and electrons on carriers

·         2 Pyruvate à 6 CO₂ + 8 NADH + 2 FADH₂ + 2 ATP

·          

§  ETC à ATP from chemiosmosis

·         Aerobic respiration: Oxygen (final electron acceptor) + electrons à water

·         Anaerobic respiration: inorganic molecules NO₃, SO₄, CO₂ final electron acceptor

·         Membrane bound structures that accept electrons and transfer them to inorganic acceptor

o   Eukaryotes – inner mitochondrial wall

o   Prokaryotes – cytoplasmic membrane

·         Forms proton gradient

o   Eletrochemical potential called proton motive force

  Chemiosmosis

§  Proton gradient à protons go through protein channels in ATP synthase à ADP and Pi form ATP

§  34 ATP per glucose

Summary of Aerobic Respiration

§1 glucose

·Glycolysis à2 ATP

·Krebs cycle à2 ATP

·Chemiosmosis à34 ATP

·Total à38 ATP

§Captures 40% of energy, rest heat

·         Fermentations

o   Release energy from sugar

o   Don’t require oxygen or ETC

o   Use organic molecule as final electron acceptor

o   Inefficient

§  1 or 2 ATP per glucose due to incomplete breakdown

o   Two stages

§  Glycolysis

§  Pyruvate into another stable organic end product like ethanol or fatty acid

o   NADH must be recycled back to NAD through fermentation since there is no ETC

o   End products

§  Species always forms same end product, but products vary by species

·         CO₂

·         Propionic acid

·         Lactic acid

·         Ethanol

·         Acetone

·         Isopropanol

·         Acetic acid

§  Identification trait

o   Ethanol

§  Yeast à alcohol, bread

o   Lactic acid

§  Lactobacilli and streptococci (important in dental carries formation) à yogurt and cheddar

§  Muscles cramp with lactic acid when no oxygen

Clinical correlation – Dental carries

o   Tooth composed of 95% hydroxyapatite

o   Dissolves at 5.5

o   Plaque convert sugar to acids

o   Streptococci make lactic acid

o   10% glucose for 1 min à pH rapidly below 5.5 within 2 min and for 22 min

§  Erosion reversible if infrequent

Critical pH=5.5. Tooth enamel in insoluable at neutral pH and begins to dissolve at 5.5

Other fermentations

o   Other products

§  Amino acids

§  Purines

§  Pyrimidines

o   Most smell bad

§  Carbs à fatty acids

§  Amino acids à amines like putracine and cadaverine and sulfur compounds

o   Anaerobic fermentation produces smelly (bad breath) and harmful acids

§  Proprionibacteria acnes on skin causes body odor

§  Ones in mouth can smell bad

Other catabolic pathways

o   Other sources of energy

§  Proteins, Carbs, Lipids

o   Must be broken down to amino acids, fatty acids, nucleotides, etc outside cell to cross membrane

o   Microbes effect breakdown by extracellular enzymes (proteases, lipases, nucleases) that hydrolyze macromolecules

§  Components transferred across membrane into Glycolysis and Krebs

§  Secretions are virulence factors due to damage cause to host tissues

·         Proteases hydrolyze proteins on eukaryotic cell surfaces like adhesins and receptors

·         Phospholipases can hydrolyze phospholipids of membranes

·         Lipase and phosphatases from intracellular pathogens can interrupt intracellular pathways

Photosynthesis

o   Basis of all life and food chains

o   Produces oxygen