Metabolism: Energy Extraction and Utilization

• Metabolism consists of energy extraction and energy utilization for biological activities like biosynthesis.
• Metabolism or intermediary metabolism refers to the reactions of energy extraction and energy use.
• Energy manipulations in cells follow basic principles:
• Molecules are degraded or synthesized stepwise via metabolic pathways.
• ATP is the energy currency.
• ATP forms through carbon fuel oxidation.
• A limited number of reaction types involving specific intermediates occur inside a cell and are common to metabolic pathways.
• Metabolic pathways are highly regulated.

Reactions and Energy

• Energy is required for muscle contraction, cell movement, active transport, and biosynthesis.
• Phototrophs capture sunlight for energy.
• Chemotrophs obtain energy by oxidizing carbon fuels.

Metabolic Pathways

• Metabolic pathways are divided into two types: catabolic and anabolic.
• Catabolic pathways:
• Combust carbon fuels to synthesize ATP.
• Fuel (carbohydrates, fats) undergo catabolism to produce CO2 + H2O + useful energy.
• Anabolic pathways:
• Use ATP and reducing power to synthesize large biomolecules.
• Useful energy + simple precursors undergo anabolism to become complex molecules.
• Amphibolic pathways can function both anabolically and catabolically.
• Anabolic and catabolic pathways may have reactions in common, but regulated, irreversible reactions are always distinct.
• Glucose, in 10 steps, turns into pyruvate, which under anaerobic conditions becomes lactate, and under aerobic conditions becomes Acetyl CoA.

Thermodynamics and Metabolic Pathways

• A thermodynamically unfavorable reaction can be driven by a favorable one.
• Metabolic pathway construction criteria:
• Individual reactions must be specific.
• The pathway in total must be thermodynamically favorable.
• Coupling a thermodynamically unfavorable reaction to a more favorable one can make it occur.

ATP as Universal Currency

• Energy from fuels or light converts into adenosine triphosphate (ATP), cells' energy currency.
• ATP hydrolysis is exergonic because the triphosphate unit contains two unstable phosphoanhydride bonds.
• ATP + H2O becomes ADP + Pi, with AG°' = -30.5 kJ mol-1 (-7.3 kcal mol-1).
• ATP + H2O becomes AMP + PPi, with AG°' = -45.6 kJ mol¯¹ (-10.9 kcal mol-1).
• ATP/ADP cycle is an essential means of energy exchange.
• Enzymes catalyze phosphoryl group transfer from one nucleotide to another.
• NMP + ATP, via nucleoside monophosphate kinase, becomes NDP + ADP.
• NDP + ATP, via nucleoside diphosphate kinase, becomes NTP + ADP.
• ATP hydrolysis shifts the equilibrium of coupled reactions, driving metabolism.
• In an endergonic reaction where A becomes B with AG°' = +16.7 kJ mol¯¹ (+4 kcal mol-1), Keq = [B]eq/[A]eq = e-AG/2.47 = 1.15 × 10-3.
• Coupling with ATP hydrolysis makes B formation exergonic: A + ATP + H2O becomes B + ADP + Pi, with AG°' = -13.8 kJ mol¯¹(-3.3 kcal mol¯¹).
• K'eq = ([B]eq/[A]eq) x ([ADP]eq[Pi]eq/[ATP]eq) = e13.8/2.47 = 2.67 x 102.

Phosphoryl Potential

• The high phosphoryl potential of ATP results from structural differences between ATP and its hydrolysis products.
• Phosphoryl-transfer potential is the standard free energy of hydrolysis.
• It is a means to compare organic molecules' tendency to transfer a phosphoryl group to an acceptor molecule.
• ATP has a higher phosphoryl-transfer potential than glycerol 3-phosphate.
• ATP + H2O becomes ADP + Pi, with AG°' = -30.5 kJ mol¯¹ (-7.3 kcal mol-1).
• Glycerol 3-phosphate + H2O becomes glycerol + Pi, with AG°' = -9.2 kJ mol-1 (-2.2 kcal mol¯¹).
• ATP has a high phosphoryl-transfer potential due to:
• Resonance stabilization.
• Electrostatic repulsion.
• Increase in entropy.
• Stabilization by hydration.
• ATP has a phosphoryl-transfer potential intermediate between high phosphoryl-potential compounds from fuel molecules and acceptor molecules requiring phosphoryl group addition.
• Creatine phosphate serves as an energy reserve in vertebrate muscle.
• Creatine phosphate + ADP becomes ATP + creatine via creatine kinase, with Keq = [ATP][creatine] / [ADP][creatine phosphate] = e -AG/2.47 = e12.6/2.47 = 162.
• In the AG°' of hydrolysis in kJ/mol (kcal/mol): --70 (16.73) for Phosphoenolpyruvate. --40 (9.56) for 1,3-Bisphosphoglycerate.
• ATP is immediate donor of free energy for biological activities, but its amount is limited.
• ATP is constantly recycled to power the cell.
• ATP goes to ADP in motion, active transport, biosyntheses and signal amplification.
• ADP goes to ATP in reactions turning oxidation of fuel molecules or photosynthesis.

Oxidation of Carbon Fuels

• Oxidation reactions include loss of electrons, coupled with reactions gaining electrons ie redox reactions.
• Carbon atoms in fuels oxidize to yield CO2, electrons accepted by oxygen to form H2O.
• The more reduced a carbon atom, the more free energy is released upon oxidation.
• Fats are a more efficient food source than glucose because fats are more reduced.
• From most to least energy:
• Methane = -820, -196.
• Methanol = -703, -168.
• Formaldehyde = -523, -125.
• Formic acid = -285, -68.
• Carbon dioxide = 0, 0.

Carbon Oxidation

• Compounds with high phosphoryl-transfer potential can couple carbon oxidation to ATP synthesis.
• The essence of catabolism is capturing the energy of carbon oxidation as ATP.
• Carbon atom oxidation forms a high phosphoryl-transfer potential compound to synthesize ATP.
• The energy released when carbon 1 of glyceraldehyde 3-phosphate is oxidized is captured first as 1,3-bisphosphoglycerate, then as ATP.

Explore metabolism, the process of energy extraction and utilization in biological activities. Learn about ATP's role as the energy currency and the principles governing energy manipulation in cells. Discover catabolic and anabolic pathways, and how energy is obtained through phototrophs and chemotrophs.