C. Catabolism of Glucose

Mechanisms of energy release: overview

1.Fermentation -- oxid of an organic compd in the absence of external e- acceptor (no O₂ req). Uses SLP (Substrate-Level Phosphorylation)

b. Entner-Doudoroff

c. Phosphoketolase

2.Respiration -- oxid of an organic compd where O₂ is the final e- acceptor. Uses ETLP (Electron Transport Level Phosphorylation) as well as SLP

3.Anaerobic respiration (unique to bacteria) -- oxid of organic compds where an external substrate other than O₂ serves as final e- acceptor. Exs: NO₃, SO₄, CO₂

1. Fermentation pathways

a. Unifying concepts for all fermentations

• organic molecules serve as both e- donors & e- acceptors
• molecule being metabolized doesn't have all its potential energy extracted from it
• NAD almost always reduced to NADH

picture of substrate level phosphorylation

Glycolysis - Sugar Lysis

b. Embden-Meyerhoff-Parnas pathway (EMP)

EMP can be divided into 2 or 3 stages

1. 6-carbon 1. Preparatory

2. 3-carbon 2. Redox

3. Prod of end products

Activation of Glucose - 6C or Preparatory

2. glc-6-phosphate is isomerized to fructose-6-phosphate & a 2nd phosphate is added

3. Now fructose-1,6-bisphosphate is unstable enough to be split

Picture of splitting of fructose-1,6-bisphosphate

1. Fructose bisphosphate aldolase (FBA) then breaks the phosphate charged fructose into 2- 3 C compds, glyceraldehyde-3-phosphate (G3P) & dihydroxyacetonephosphate (DAP)

Energy Extraction- 3C or Redox

1. DAP must be converted into G3P

2. Inorganic phosphate (P_i) is added to G3P to make 1,3-bisphosphoglycerate (BPG)

3. After several enzymatic rearrangements, the final product of EMP is pyruvate

Picture of remaining stages of EMP

Lots of energy released (~ 100 kcal)

Some - temporarily trapped in e- (+ H⁺) as NADH

Some - P_i to the 3-C molecule, forming 1,3-BPG

Rest is released as heat

Total energy of glc available by complete oxidation = 688 kcal/mole.

Total energy yield of 2 net ATP = 2 x 7.3 = 14.6 kcal/mole.

Fermentation through EMP is ~ 2% efficient compared to complete oxidation by aerobic respiration

98% of energy potentially available in glc is not available to a fermenting cell

Summary of EMP

The net energy gain is 2 ATP per glc

Production of end products

Picture of fate of pyruvate

c. Entner-Doudoroff

Picture of ED pathway

Summary of Entner-Doudoroff

• only 50% as efficient in ATP prod as glycolysis
• formation of NADPH
• an impt mechanism for prod NADPH and 3C building blocks used in biosynthetic pathways

d. Phosphoketolase

• heterolactic fermenters

picture of phosphoketolase pathway