Microbial Metabolism

Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives You should be able to:

1. Describe the three stages of aerobic glucose catabolism (glycolysis, the Krebs cycle, and the electron transport chain).

2. Compare the pentose phosphate pathway and the Entner-Doudoroff pathway with glycolysis in terms of energy production and products.

3. Describe fermentation and contrast it with respiration.

4. Identify useful end-products of fermentation, and explain how fermentation reactions are used in the identification of bacteria.

Obtaining Energy

• Autotrophy – self feeding; utilize CO21. photoautotrophs2. chemoautotrophs (oxidize inorganic subst., nitrites and sulfides)

• Heterotrophy – other feeding; use organic molecules1. photoheterotrophs (chemical energy from light)2. chemoheterotrophs (chemical energy from breaking down organic materials)

Carbohydrate Catabolism• Polysaccharide hydrolysis to

monosaccharides then to glucose• Glycolysis

– 2 pyruvic acid, 2 ATP, 2 NADH (Substrate level phosphorylation)

• Either via 1. Cellular respiration – CO2 and water

waste products); with 38 ATP’s2. Fermentation – acid; alcohol (organic

waste products); with 2 ATP’s

Alternate Pathways to glycolysis

• Pentose phosphate (phosphorylated 5 Carbon sugar – ribulose, xylulose, ribose)

• Produce precursor metabolites

- used in synthesis of nucleotides, amino acids, and glucose by photosynthesis

• 1 ATP, 2 NADPH

Alternate Pathways to glycolysis

• Entner-Doudoroff

- used by gm. - Pseudomonas aeruginosa; gm. + Enterococcus faecalis

• Catabolize glucose to pyruvic acid

• Net 1 ATP, also yield precursor metabolites and NADPH, not produced by pentose phosphate pathway

Cellular respiration• Aerobic respiration – total 38 ATP• Synthesis of acetyl-CoA – lose a CO2; gain NADH• Krebs cycle/citric acid cycle/TCA cycle (cytoplasm

in prokaryotes) – net 2 ATP, 6 NADH, 2 FADH2, 4 CO2

• Electron transport chain (in mitochondria)

- passing e- to final e- acceptor (H2O)

- NADH (yield 3 ATP) and FADH2 (yield 2 ATP) • Summary: glucose + 6 O2 → 6 CO2 + 6 H2O + energy (38

ATP)• Anaerobic respiration (reduction of sulfate, nitrate,

carbonate, and give off methane)

Fermentation • Partial oxidation of sugar to release energy

(oxidize NADH to NAD+)• Summary:

Glucose → 2 Lactic acid + 2 ATP

Glucose → 2 ethanol + 2 CO2 + 2 ATP• Some useful in health and industry • Others are harmful

- Clostridium perfringens results to gangrene

- wine spoilage (acetic/lactic acid)

Homework 1. Define terms: autotrophy, heterotrophy, aerobic

respiration, anaerobic respiration, Entner-Doudoroff pathway, and pentose phosphate pathway.

2. Describe glycolysis, kreb cycle, and electron transport chain.

3. What intermediate (chemical) product is needed to enter through the kreb’s cycle?

4. In electron transport of oxidative phosphorylation, what are the electron carriers? What is the final electron acceptor?

5. What process is common to both cellular (aerobic) respiration and fermentation (non-aerobic)?

6. Describe lactic acid fermentation and ethanol fermentation.

7. List 4 useful fermentation products.