metabolic diversity 2: catabolism

polysaccharides are catabolized into

• pyruvate

• monosaccharides

• enters glycolytic pathway

lipids and amino acids are catabolized into

• glycerol

• acetate

• components will enter glycolysis or TCA cycle

aromatic compounds like lignin and benzoate derivatives are catabolized to

acetate via catechol pathway → enters TCA cycle

microbial syntrophy

• cooperative interaction between two or more microbial species to degrade a single substrate

• often used for aromatic and short-chain waste products of petroleum (contamination)

• usually substrate is very stable and has a + delta G

• coupling metabolism with more than one organism → reaction can become favorable (- delta G)

three primary catabolic strategies

• fermentation

• respiration

• photoheterotrophy

fermentation

• partial breakdown of organic food w/out transfer of electrons to inorganic terminal electron acceptor

• electrons return to products of pyruvate - no net change in redox state

• forms partly oxidized fermentation products

• generates ATP mainly through substrate-level phosphorylation

respiration

• aerobic or anaerobic depending on terminal electron acceptor (aerobic = oxygen, anaerobic = nitrate, sulfate)

• obtains more energy

• electron transport system

• complete oxidation of glucose to CO2

photoheterotrophy

use light absorption from chlorophyll to drive photolysis of organic molecules

catabolism of glucose involves three main routes

1. glycolysis (embden-meyerhof-parnas EMP pathway)

2. entner-doudoroff ED pathway

3. pentose phosphate PPP pathway or pentose phosphate shunt

EMP pathway (glycolysis)

1. glucose activation step: glucose phosphorylated by 2 ATP into fructose 1,6 bisphosphate → cleaved into 2 glyceraldehyde 3-phosphate G3P

2. energy-yielding step: 2 G3P → redox reactions to oxidize compound, ATP synthesis → 2 pyruvate

net output of EMP/glycolysis

• 2 ATP

• 2 NADH

• 2 pyruvate

entner-doudoroff ED pathway

• glucose phosphorylated by ATP→ activated

• NADP to NADPH to generate 6-phosphogluconate (dehydrogenation)

• removal of water (dehydration) → cleaved → 2 glyceraldehyde 3-phosphate G3P

• G3Ps enter glycolysis to form pyruvate

net output of ED pathway

• 1 ATP

• 1 NADH

• 1 NADPH

• 1 pyruvate

pentose phosphate pathway

• generates NADPH and sugar phosphate molecules for biosynthesis

• glucose → phosphorylated to glucose 6-phosphate → dehydrogenated to 6-phosphogluconate

• converted into intermediate ribulose-5-phosphate which produces 3C, 4C, 6C, 7C sugars

• sugars can be used in EMP/glycolysis to form pyruvate, others used in biosynthesis

net output of pentose phosphate pathway

• 1 ATP

• 2 NADPH

• 3C, 4C, 6C, 7C sugars