Stegmann

Ability to do work

Measure of the disorder or randomness of a system

• the direction of a reaction can be predicted by a thermodynamic qunatity called gibbs free energy change ∆G

• ∆G inculdes entalpy & entropy

∆G = ∆H - T∆S

• ∆H: change in entalpy, the heat energy absorbed or released; T∆S: product of temperature & entropy change

• If ∆G < 0 the process may go forward

• If ∆G > 0 the reaction will go in reverse

∆G0:

• temperature: 298K (25°C)

• Pressure 1 atm

• Concentration 1M

• In living cells additional standard condition of pH 7

The additivity of energy change is central to all living metabolism

Additivity makes it possible to “do work” by coupling an energy-yielding reaction to an energy-spending one

A reducing agent

An oxidizing agent

= Adenosine triphosphate

• contains a base, sugar and three phosphates

• Under physiological conditions ATP always forms a complex with Mg2+

• ATP is a “medium size” energy carrier, because the cell contains many phosphorylated molecules that yield greater energy upon hydrolysis

• ATP can transfer energy to cell processes in three different ways

1. hydrolysis releasing phosphate (Pi)

2. Hydrolysis releasing pyrophosphate (PPi)

3. Phosphorylation of an organic molecule

• besides ATP other nucleotides carry energy

= Nicotinamide adenine dinucleotide

• Carries two or three times as much energy as ATP

• It also donates & acceptors: NADH is the reduced form, NAD+ is the oxidized form

• Reduction od NAD+ consumes two hydrogen atoms to make NADH

NAD+ + 2H+ + 2e- -> NADH + H+ ∆G0’ = 62kJ/mol

• reaction requires energy input from food molecules

= Flavine adenine dinucleotide

• is another related coenzyme that can transfer electrons

• FADH2 (reduced form) versus FAD (oxidized form)

• Unlike NAD+ FAD is reduced by two electrons & two protons

Enzymes catalyze biological reactions

• lower the activation energy (Ea) allowing rapid conversion of reactants to products

• Bring reactants close together & remove surrounding water

• Enzymes couple specific energy-yielding reactions with energy-requiring reactions

• An enzyme may also have an allosteric site

Polysaccharides such as starch, cellulose and pectin are hydrolyzed to glucose

Lipids & amino acids are catabolized to gylcerol & acetate

Complex aromatic molecules such as lignin & halogenated aromatic pollutants & benzoate derivatives are broken down to acetate & other molecules

Peptides are hydrolyzed to amino acids then broken down to acetate, amines & other molecules

Carbohydrates are broken down by specific enzymes to disaccharides and then to monosaccharides such as glucose. Glucose and sugar acids are converted to pyruvate which releases acetyl groups. Acetyl groups or acetate are also the breakdown products of fatty acids, amino acids and complex aromatic plant materilas such as lignin

Partial breakdown of organic food without electron transfer to an inorganic terminal electron acceptor

The hydrogens from NADH + H+ are transferred back onto the products of pyruvate forming partly oxidized fermentation products

Most fermentations do not generate ATP ebyond that preduced by substrate lever phosphorylation: microbes compensate for the low efficiency of fermantation by consuming large quantities of substrate & excreting large quantities of products

Complete breakdown of organic molecules with electron transfer to a terminal electron acceptor such as O2

Catabolism is conducted with a “boost” from light

Produces two molecules of lactic acid

Produces two molecules of ethanol & two CO2

Produces one molecule of lactic acid, one ethanol and on CO2

Produces acetate, formate, lactate & succinate as well as ethanol, H2 & CO2

1. Lactobacillus ferments lactose into lactic aid

2. Propionibacterium converts lactate to propionate, acetate & CO2

= tricarboxylic acid cycle/Krebs cycle/Citric acid cycle

• Glucose catabolism connects with the TCA cycle through pyruvate breakdown to acetyl-CoA & CO2: Acetyl-CoA enters the TCA cycle by condensing with the 4-C oxaloacetate to form citrate

• Conversion of pyruvate to acetyl-CoA is catalyzed by a very large multisubunit enzyme called pyruvate dehydrogenase complex (PDC)

Pyruvate + NAD+ + CoA -> Acetyl-CoA + CO2 + NADH + H+

• PDC activity is a key control point of metabolism induced when carbon sources are plentiful & repressed under carbon starvation & low oxygen

• The acetyl-CoA formed can then enter the TCA cycle

• TCA cycle also serves as an anabolic function: alpha ketoglutarate is used to make certain amino acids; Oxaloacetate is aminated to form aspartate, entering pathways to purines & pyrimidines as well

Products of the TCA cycle for each pyruvate oxidized

• 3CO2 (produced by decarboxylation)

• 4NADH & 1 FADH2 are produced by redox reactions

• 1ATP is produced by substrate level phosphorylation (some cells make GTP instead - equivalent in stored energy)

-> times 2 for one molecule of glucose (1 glucose -> 2 pyruvate)

• after the completion of the TCA cycle all the carbons of glucose have been released as waste CO2, however the metabolic pathway is not complete until the electrons carried by the coenzymes (NADH/FADH2) are donated to a terminal electron acceptor

• The overall process of electron transport & ATP generation is termed oxidatice phosphorylation

When glucose is abscent cells can can catabolize acetate or fatty acids using a modified TCA cycle: Gyloxylate shunt

• consists of two enzymes that divert isocitrate to glyoxylate & incorporate a second acetyl-CoA to form malate

• Catabolism of aromatic molecules by bacteria & fungi recycles lignin & other important substances within ecosystems: toxic pollutants are also degraded

• Benzoate & related compounds undergo aerobic catabolism to catechol: Catechols are degraded through several alternative pathways to the TCA cycle

= electron transport system

• The ETS is embedded in s membrane that seperates two aqueous compartments: maintains the ion gradient generated by the ETS

Electron transport proteins

• oxidize one substrate (removing electrons) & reduce another (donating electrons) -> they couple different half reactions of the electron tower

• Oxidoreductases consist of multiple protein complexes that include cytochormes as well as noncytochrome proteins (cytochromes are colored proteins whose absorbance spectrum shifts when there is a change in redox state

• Work in a ETS

• The transfer of H+ through a proton pump generates an electrochemical gradient of protons

• It drives the conversion of ADP to ATP through ATP synthase: chemisomotic theory

1. The substrate dehydrogenase recieves a pair of electrons from an organic substarte such as NADH or an inorganic substrate such as H2

2. H donates the electrons ultimately to a mobile electron carrier such as quinone: quinone picks up 2H+ from solution & is thus reduced to quinol

3. The oxidation of NADH & reduction of Q is coupled to pumping 4 H+ across the membrane

4. A terminal oxidase complex which typically includes a cytochrome recieves the two electrons from quinol (QH2), the 2 H+ are translocated outside the membrane, in addition the transfer of the two electrons through the terminal oxidase complex is coupled to the puping of 2H+

5. The terminal oxidase complex transfers the electrons to a terminal electron acceptor such as O2: each oxygen atoms recieves two electrons & combines with two protons from the cytoplasm fo form one molecule of H2O

1/2 O2 + 2H+ -> H2O

In E. Coli ETS can pump up to 8H+ for each NADH molecule & up to 6H+ for each FADH2 molecule

Highly conserved protein complex, made out of two parts

• F0: embedded in the membrane; pumos protons

• F1: ptrotrudes in the cytoplasme; generates ATP