Chapter 8

 

Microbial Metabolism

 

 

Metabolism

Chemical reactions used for biosynthetic and energy-harvesting processes

Anabolism - utilizes energy for biosynthesis of compounds

Catabolism - releases energy during breakdown of compounds

 

 

Components of Metabolic Pathways

Enzymes

protein catalysts that accelerate chemical reactions by lowering the activation energy

bind to substrates at the active site

often require specific cofactors to function

  organic cofactors (a.k.a. coenzymes) come from vitamins

  inorganic cofactors include Fe, Zn, Mg, & Cu

 

 

Factors Affecting Enzyme Activity

Denaturation - causes distortion of enzymes shape and prevents binding of the substrate to the active site

Temperature

enzymes function best in a specific temperature range

  if temp. is too high- the reaction takes place too quickly resulting in denatured/useless products

  if temp. is too low- the reaction takes place too slowly resulting in insufficient amounts of the product

pH

prefer pH between 5 and 8 (slightly acidic-slightly basic)

  if too high or too low, results in denatured/useless products

Salt Concentration

prefer isotonic or hypotonic environments

  if hypertonic the result is denatured/useless products

Enzyme Inhibitors - chemicals that inhibit enzyme function

may be reversible or irreversible

 

 

Enzyme Inhibitors

Competitive - inhibitor binds to active site of enzyme and obstructs access to that site by substrate

Non-competitive - inhibitor and substrate bind to different sites on enzyme and cause shape to change

i.e. regulatory molecules for negative feedback

 

Energy

Cells manage energy through chemical reactions involving breaking bonds and transferring electrons

exergonic reactions - release energy

endergonic reactions - require energy to proceed

 

 

Oxidation-reduction (redox) Reactions

One or more electrons are transferred from one substance to another

oxidation = loss of electrons

   dehydrogenation - loss of hydrogen

reduction = gaining of electrons

   hydrogenation - gain of hydrogen

             

Electron Carriers

Shuttle electrons to facilitate the transfer of redox energy

Electron donor loses electron (oxidation) which is taken up by an electron acceptor (reduction)

Electron is usually part of hydrogen atom

Energy is released every time electron is transferred 

Electrons must be transferred to a final electron acceptor

Three types of carriers are coenzymes:

Nicotinamide adenine dinucleotide (NAD+)

Flavin adenine dinucleotide (FAD)

NAD phosphate (NADP+)

The reduced forms are NADH, FADH2, and NADPH

 

 

Adenosine Triphosphate (ATP)

Primary energy currency of the cell

Composed of an adenine, ribose, and 3 phosphate groups (PO4-)

Energy is stored in the high-energy phosphate bonds and released when they are broken

ATP ADP + P (releases energy)

ADP + P ATP (requires energy)

 

 

Aerobic Respiration

The complete oxidation of glucose to CO2

C6H12O6 + 6O2 6CO2 + 6H2O + 38 ATP

Step 1 – Glycolysis

glucose 2 pyruvate (pyruvic acid)

  2 ATP are used

  4 ATP are produced

  2 NADH are produced

Step 2 - pyruvate to acetyl Co-A (Transition Step)

occurs twice for every glucose molecule

breaks down pyruvate to a 2 carbon molecule

  one O2 is used for each pyruvate

  one CO2 is produced for each pyruvate

  one NADH is produced for each pyruvate (net gain of two)

Step 3 - Krebs cycle

occurs twice for every glucose molecule

results in the oxidation of the last 2 carbon atoms of glucose

acetyl Co-A binds with oxaloacetic acid to form citric acid

citric acid then progresses through a series of reactions resulting in the reformation of oxaloacetic acid

  C from acetyl Co-A is oxidized creating two CO2 per cycle

  1 ATP is produced per cycle

  3 NADH are produced per cycle

  1 FADH2 is produced per cycle

Step 4 - Electron Transport Chain

group of electron carriers that pass electrons sequentially from one to another (series of redox reactions)

uses 10 NADH and 2 FADH2 from glycolysis, transition step, and Krebs cycle to synthesize ATP

protons are pumped out of the cell to generate proton motive force

  used to transport substances into or out of cell, drive flagella, etc.

ATPsynthase - harvests energy from proton motive force to  drive the synthesis of ATP

  each NADH has enough energy to produce 3 ATP and each FADH2 can produce 2

  30 ATP from NADH and 4 from       FADH2

O2 is the final electron acceptor

 

 

Anaerobic Respiration

Less efficient form of energy transformation than aerobic respiration

Identical to aerobic respiration except O2 is replaced with another inorganic molecule as final electron acceptor

ex. Nitrate (NO3-), sulfate (SO4-2)

 

 

Fermentation

Incomplete oxidation of glucose

Follows glycolysis when O2 is absent

Pyruvate is broken down and NADH is converted back to NAD+

Only ATP produced is from glycolysis

Alcohol fermentation

results in 2 ATP, CO2, and an alcohol (usually ethanol)

Acidic fermentation

results in 2 ATP plus an acid such as lactic acid and butyric acid

 

 

Biosynthesis of Macromolecules

Metabolites - products of metabolism that can be used in anabolic pathways

metabolites from glycolysis, Transition step, and Krebs cycle can be used to make amino acids, lipids, nucleic acids, and carbohydrates

 

Amphibolism

Catabolic and anabolic pathways are integrated to improve cell efficiency

Gluconeogenesis – pyruvate is used to make glucose when supplies are low

Amination – products from Krebs cycle are used to synthesize amino acids

  transamination occurs when amino acids and carbohydrates are interchanged

Deamination – amino acids are converted to intermediates of the Krebs cycle when carbohydrate supplies are low

Beta-oxidation – fatty acids can be used to make acetyl CoA

 

 

Photosynthesis

Carried out by photoautotrophs

Utilizes light energy to synthesize glucose from CO2 and water

6CO2 + 6 H2O C6H12O6 + 6O2

Photosynthetic pigments absorb light and use it to energize electrons

Chlorophylls - found in plants, algae, and cyanobacteria

Bacteriochlorophylls - found in purple and green bacteria

  absorb different types of light than chlorophylls allowing these bacteria to live in different environments

Involves 2 reactions similar to electron transport chain:

Light-dependent reactions

  proceed only in the presence of sunlight

  energy producing catabolic reactions

  light activates chlorophyll and energy is used to split water into oxygen and hydrogen

  produce ATP and NADPH

Light-independent reactions (Calvin cycle)

  proceed regardless of sunlight

  anabolic reactions for synthesis of glucose

  utilize ATP and NADPH from light reaction

  carbon atoms from CO2 are fixed to the carbon backbones of organic molucules