(6) CH3 - Microbial metabolism PT2

Lecture 10

cells conserve energy released from exergonic reactions by

coupling the reaction to the biosynthesis of energy-rich compounds

reactions that release sufficient energy to form

ATP require oxidation-reduction biochemistry

oxidation =

removal of an electron (or electrons) from a substance

reduction =

addition of an electron (or electrons) to a substance

OIL RIG

oxidation is loss of electrons, reduction is gain or electrons

redox reactions occur

in pairs

the substance oxidized is called the

electron donor

the substance reduced is called the

electron acceptor

many electron donors exist in nature

organic and inorganic compounds

many electron acceptors also exist

nitrogen compounds, sulfur compounds, and organic compounds

many substances can either be electron donors or electron acceptors depending on

the substance they couple with in an redox reaction

the constituents of each side of the arrow in half reactions are called

a redox couple

substances differ in their

tendency to donate or accept elections

reduction potentials for half reactions are written as

reductions at pH 7

the redox tower represents the

range of possible redox couples in nature

substances toward the top of the redox tower

(reduced; more negative E0) prefer to donate electrons (become oxidized)

substances toward the bottom of the redox tower

(oxidized; more positive E0) prefer to accept electrons (become reduced)

redox tower - the farther electrons “drop” (the larger the difference in reduction potential between electron donor and electron acceptor), the

greater the amount of free energy released during the redox reaction

the ΔG0 of a redox reaction can be calculated three ways:

1. by knowing the free energy of formation values

2. by knowing the equilibrium constant

3. by knowing the difference in the reduction potential of the two half reactions that make up the full redox reaction

redox tower - oxygen (O2)

bottom of the redox tower, strongest electron acceptor of significance in nature

redox tower - glucose

top of the redox tower, strong electron donor

redox tower - redox couples in the middle of the tower can

serve as electron donors or electron acceptors depending on who they interact with

redox reactions are typically

facilitated by coenzymes that associate with the redox enzymes that catalyze the reaction

NAD+ is a very common

redox coenzyme (reduced form is NADH)

• NADH is good electron donor, NAD+ is a weak electron acceptor

electron shuttling mediated by NAD+/NADH is common in

microbial catabolism

NADP+ and its reduced form (NADPH) are made from

NAD+ and NADH by adding a phosphate molecule to the 2’ OH of the ribose

NADP+/NADH participate in

anabolic redox reactions *biosynthesis of cellular precursors)

NAD+/NADH participate in

catabolic redox reactions (breakdown of organic molecules to release energy and/or generate small molecules for sue in anabolic reactions)

chemical energy released from redox reaction is

conserved/stored and used later to fuel energy-requiring cell functions

this energy is stored in compounds that contain

energy-rich phosphate or sulfur bonds (called high-energy bonds)

biosynthesis os these compounds functions as a

free-energy “trap” and their hydrolysis releases the stored energy to drive endergonic reactions

• however, not all phosphate bonds are high-energy bonds

high-energy bonds are

covalent bonds whose breakdown by water (hydrolysis) is accompanied by a decrease in free energy (exergonic, favorable reaction)

compounds that conserve energy in microbial metabolism via their high-energy bonds include;

• ATP (PRIME ENERGY CURRENCY)

• phosphoenolpyruvate

• coenzyme A derivative

ATP is a very dynamic molecule in the cell; it is

continuously broken down to drive anabolic (biosynthetic) reactions and is resynthesizes with energy derived from catabolic redox reactions

for long-term energy storage, microbes typically produce

insoluble polymers that can be broken down later to generate ATP

long-term energy storage - prokaryotes

• glycogen (poly glucose)

• PHA and PHB

• elemental sulfur (S)

long-term energy storage - eukaryotes

• starch (poly glucose)

• lipids (simple fats)

fermentation and respiration are major

catabolic pathways that result in energy conservation in chemoorganotrophs

fermentation

• form an anaerobic catabolism

• uses organic compounds as both electron donors and electron acceptors

• produces ATP via substrate-level phosphorylation

respiration

• form of aerobic or anaerobic catabolism

• organic or inorganic electron donors are oxidized

• O2 (aerobic respiration) or another compound (anaerobic respiration) function as terminal electron acceptors

• typically produces ATP via oxidative phosphorylation

glycolysis

glucose is oxidized to pyruvate to generate ATP

respiration after glycolysis

pyruvate is further oxidized to CO2 in TCA cycle

fermentation after glycolysis

private is used an electron acceptor to achieve redox balance in glycolysis

two of the reactions in glycolysis are redox reactions

• Free energy is released and conserved by the simultaneous
  production of energy-rich compounds (1,3-bisphosphoglyceric
  acid and phosphoenolpyruvate)

• Energy-rich phosphate bonds in compounds are transferred to
  ADP via substrate-level phosphorylation to generate ATP

golysis stage 1 = “preparatory” reaction: consumes 2 ATP

• phosphorylates glucose, then splits glucose

• generating 2 molecules of G3P

glycolysis stage 2 = ATP-generating reactions: 2 ATP, 2 pyruvate, 2 NADH

• Oxidation of G3P via reduction of NAD+ to NADH (exergonic)

• Coupled to endergonic reaction in which inorganic phosphate is
  transferred to G3P (G3P dehydrogenase

• Produces high energy intermediates (1-3 phosphoglycerate and
  phosphoenolpyruvate) used to generate ATP via substrate-level
  phosphorylation

glycolysis stage 3 = formation of fermentation products

NADH oxidized back to NAD+ (needed to continue to run glycolysis)

glycolysis stage 3 - fermentation achieves redox balance

2 molecules private reduced by an NADH-containing enzyme to fermentation products

• regenerates 2 NAD+ from 2 NADH

• lactic acid bacteria: pyruvate reduced to lactate

• yeast: private reduced to ethanol and CO2

fermentative diversity

• Many different types of fermentation (and products)

• Not all compounds are fermentable

• Sugars are highly fermentable, sugars/polysaccharides other
  than glucose are first converted to glucose via enzymatic
  reactions performed by various microbes

• Different types of fermentations are classified by either the
  substrate fermented or the products formed

some fermentations allow for additional ATP synthesis by substrate-level phosphorylation

• Possible if the fermentation product is a fatty acid because the fatty
  acid is formed from its coenzyme-A precursor

• Coenzyme-A derivatives (e.g. acetyl-CoA) contain high-energy
  bonds which can be used to generate ATP from ADP

cells that can perform both

fermentation and respiration will choose the pathway that most energetically benefits them

products of fermentation

ethanol, lactic acid, etc.

products of respiration

CO2

microbes use up more of glucose’s potential for energy production if

they metabolize it via respiration (get more energy)

• thus, when O2 is available cells will choose respiration

during respiration, glucose is first

catabolized via glycolysis, but instead of reducing pyruvate to fermentation products (and discarding them) pyruvate is fully oxidized to CO2 via the citric acid cycle and glyoxylate cycles