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The ruby-throated hummingbird can store enough fuel to fly 500 miles across the Gulf of Mexico A prodigious feat of metabolism,
How does a cell extract energy and reducing power from its environment? How does a cell synthesize building blocks and macro-molecules?
The network of chemical reactions has a coherent design containing many common motifs For example, glucose metabolism is conserved from bacteria to humans Catabolism Anabolism
Fuel (Carbohydrates, fats) CO 2 + H 2 O + energy Catabolism Energy + simple precursors complex molecules Anabolism
Metabolism is composed of many coupled interconnecting reactions
Coupled reactions allow thermodynamically unfavorable reactions to proceed as long as the sum of the free energy changes of coupled reactions is negative. A B + C G 0 ’= +21 kJ mol -1 B D G 0 ’= -34 kJ mol -1 A C + D G 0 ’= -13 kJ mol -1
ATP is the universal currency of free energy in biological systems
The high phosphoryl transfer potential of ATP results from structural differences between ATP and its hydrolysis products Resonance stabilization of Pi and ADP Electrostatic repulsion, triphosphate of ATP has 4 negative charges at pH 7 Stabilization due to hydration, ADP and Pi can bind more water than ATP
Activated carrier of e- for reductive biosynthesis NADPH
Activated carrier of two-carbon fragments Coenzyme A
Acyl groups are linked to CoA by thioester bonds
Hydrolysis of thioester is thermodynamically more favorable than that of oxygen ester because e- of the C=O bond cannot form resonnance structures with the C-S bond Consequently, Acetyl CoA has high acetyl-group transfer potential Acetyl CoA + H 2 O Acetate + CoA + H + G 0 ’= -31.4 kJ mol -1
Use of activated carriers illustrates 2 key aspects of metabolism : 1. Kinetic stability in the face of large thermodynamic driving force for reaction: NADH, NADPH and FADH 2 react slowly with O 2 in absence of catalyst ATP and Acetyl CoA react slowly with H 2 O in absence of catalyst 2. Most interchanges of activated groups are accomplished by a rather small set of carriers (Table 15) = conservation and unifying motifs of biochemistry as well as modular design
ADP a toutes les sauces: Co-enzymes may have evolved from early RNA catalysts