Vitamins Bioenergetics - Vitamins and Cofactors

Vitamins & Cofactors Reaction mediated Human Deficiency Disease Beriberi Pellagra ① ② Acyl-group transfer Re-dox ③ Re-dox – ④ ⑤ Acyl-group transfer Amino-group transfer dermatitis ⑥ Alkylation Pernicious anemia ⑦ ⑧ ⑨ Carboxylation Acyl-group transfer C1-group transfer Megaloblastic anemia Re-dox Scurvy 10 Ascorbic acid (vitamin C) Fat-soluble ① ② ③ ④ Ascorbate Retinal – – – Night blindness Rickets – Hemorrhage 1 Vitamins & Cofactors ① Vitamin A Light absorption ③ Vitamin E Redox Fat Soluble ② Vitamin D3 Calcium homeostasis ④ Vitamin K Blood clotting Vitamins & Cofactors 2 Vitamins & Cofactors Overview of Metabolism 3 Recall from First lecture: Energetics of Life Chemistry of Life: Biochemistry Chaos ⇌ Cell DSsystem is ⊝ DSuniverse = DSsystem + DSsurroundings ⊕ ⊝ ⊕ • As the entropy of the universe increases, creating and maintaining order requires work and energy. • Living organisms exist in a dynamic steady state and are never at equilibrium with their surroundings. • Energy coupling allows living organisms to transform energy, releasing much of it to the surroundings. Metabolism • Living organisms cannot create energy from nothing. • Living organisms cannot destroy energy into nothing. • Living organism may transform energy from one form to another. • In the process of transforming energy, living organisms must increase the entropy of the universe. • In order to maintain organization within themselves, living systems must be able to extract useable energy from their surroundings and release useless energy (heat) back to their surroundings. 4 Organisms Use the First Law Big-Time (perform energy transformations) to Stay Alive Favorable and Unfavorable Reactions • – Their cellular concentration is far higher than their equilibrium concentration. – Metabolites, such as ATP, NADH, NADPH, can be synthesized using the energy from sunlight and fuels….. DS > 0 DG°< 0 The breakdown of some metabolites releases a significant amount of energy (exergonic). • catabolism Synthesis of complex molecules and many other metabolic reactions requires energy (endergonic). – A reaction might be thermodynamically unfavorable (DG°> 0). DG°> 0 • Creating order requires work and energy. anabolism Metabolism Issues: • Thermodynamics and biochemistry; carbon/oxygen cycle & nitrogen cycle • Common organic-chemical principles in biochemistry • Some biomolecules are “high energy” with respect to their hydrolysis and group transfers. • Energy stored in reduced organic compounds can be used to reduce cofactors such as NAD+ and FAD, which serve as universal electron carriers and lead to ATP formation. 5 Metabolism • Carbon/oxygen cycle + • Recall that living organisms are built of complex structures. • Building complex structures means paying a huge entropic price; need a constant input of energy. • The ultimate source of this energy on Earth is sunlight Metabolism Metabolism is the Sum of All Chemical Reactions in the Cell NH3 6 Metabolism In biochemistry, the oxidation of fuels Metabolism is the Sum of All–Chemical Reactions inreduced the Cell – – with O2 is stepwise and controlled. Catabolism is convergent and Anabolism is divergent Recall that being thermodynamically favorable is not the same as being kinetically rapid. NH3 Metabolism NAD and NADP Are Common Redox Cofactors • These are commonly called pyridine nucleotides. • They can dissociate from the enzyme after the reaction. • In a typical biological oxidation reaction, hydride from an alcohol is transferred to NAD+, giving NADH. 7 Metabolism THE ATP CYCLE This “buffering” of energy in the cell keeps the [ATP] high Energy enough to keep fighting the second law of thermodynamics. Charge = + ½ [ADP-3] [ATP-4] + [ADP-3] + [AMP-2] [ATP-4] Metabolism + H2PO4– + H2 O Adenosine diphosphate (ADP) If this reaction is allowed to come to equilibrium, what is the DG? + HPO42– + H+ (at pH 7) the actual free-energy change of a reaction depends on: – A constant (the standard change in free energy, DG°) – actual concentrations of products and reactants – For the reaction aA + bB ⇌ cC + dD: c [In biochemistry, we add a prime (′) because we pull [H+] out of the expression and set it to 10-7, not 1 M] d [C ] [ D] DG′= DG'°′ ° + RT ln [ A]a [ B]b 8 Metabolism + HPO4–2 + H+ + H2O (at pH 7) Adenosine diphosphate (ADP) DG′= DG'°′ ° + RT ln [C ]c [ D]d [ A]a [ B]b [C ]c [ D]d DG′= DG°′ '° + RT ln K′eqa b d ] [ B] [C ]c [ D[]A DG = DG°′ '°=+-RT ln K′eqa b [ A] [ B] TABLE 13-2 Relationship between Equilibrium Constants and Standard FreeEnergy Changes of Chemical Reactions DG' DG°′= DG°ʹ= –7.3 kcal/mole kcal/mole TABLE 13-3 Relationships among K'eq, DG'˚, and the Direction of Chemical Reactions When K'eq is . . . >1.0 1.0 <1.0 DG'˚ is . . . Starting with all components at 1 M, the reaction . . . negative proceeds forward zero is at equilibrium positive proceeds in reverse But, what is the actual DG′ of ATP Hydrolysis IN THE CELL? (kcal/mol)a K'eq (kJ/mol) 103 –17.1 102 –11.4 –2.7 101 –4.1 –5.7 –1.4 1 0.0 0.0 10–1 5.7 1.4 10–2 11.4 2.7 10–3 17.1 4.1 10–4 22.8 5.5 10–5 28.5 6.8 10–6 34.2 8.2 Although joules and kilojoules are the standard units of energy and are used throughout this text, biochemists and nutritionists sometimes express DG'˚ values in kilocalories per mole. We have therefore included values in both kilojoules and kilocalories in this table and in Tables 13-4 and 13-6. To convert kilojoules to kilocalories, divide the number of kilojoules by 4.184. a Metabolism + HPO4–2 + H+ + H2O (at pH 7) Adenosine diphosphate (ADP) Go back the original equation: [MgADP- ] × [Pi ] DG = DG °'+ RT ln [MgATP2- ] At 25 °C: DG°′ of ATP Hydrolysis is Mg++ Dependent DG′= DG'°′ ° + RT ln [ADP-3] -2] [HPO4 [ATP-4] [ADP-3] [HPO4-2] + 0.59ln [ATP-4] DG′ = DG°′ + RT ln DG′ = DG°′ [C ]c [ D]d [ A]a [ B]b In RBCs = 1.5 x 10-4 M DG′ = DG°′ + 1.36log (1.5 x 10-4) DG′ = (-7.3) + (-5.2) DG′ = -12.5 kcal/mole WOW! Other than concentrations, what makes the DG′ of ATP so high? 9 Metabolism • Contributions of the incredible favorability of the ATP hydrolysis reaction: – Better charge separation in products (relief of charge repulsion) – More favorable resonance stabilization of products ❸ H2PO4– – Ionization of products (ADP or Pi) ⇌ HPO + H 42– + (at pH 7) Are there other compounds in the cell with high negative DG°′ of hydrolysis? Metabolism DG°′= –12 kcal/mole TABLE 13-4 Standard Free-Energy Changes of Some Chemical Reactions D G'˚ Reaction type 1,3-bisphosphoglycerate DG°′= –11.8 kcal/mole DG°′= –10.3 kcal/mole (kJ/mol) (kcal/mol) Hydrolysis reactions Acid anhydrides Acetic anhydride + H2O à 2 acetate ATP + H2O à ADP + Pi ATP + H2O à AMP + PPi PPi + H2O à 2Pi UDP-glucose + H2O à UMP + glucose 1-phosphate –91.1 –30.5 –45.6 –19.2 –43.0 –21.8 –7.3 –10.9 –4.6 –10.3 Esters Ethyl acetate + H2O à ethanol + acetate Glucose 6-phosphate + H2O à glucose + Pi –19.6 –13.8 –4.7 –3.3 Amides and peptides Glutamine + H2O à glutamate 1 NH4+ Glycylglycine + H2O à 2 glycine –14.2 –9.2 –3.4 –2.2 Glycosides Maltose + H2O Lactose + H2O –15.5 –15.9 –3.7 –3.8 –7.3 –1.7 –1.7 –0.4 à à 2 glucose glucose + galactose Rearrangements Glucose 1-phosphate à glucose 6-phosphate Fructose 6-phosphate à glucose 6-phosphate Elimination of water Malate à fumarate + H2O Oxidations with molecular oxygen Glucose + 6O2 à 6CO2 + 6H2O Palmitate + 23O2 à 16CO2 + 16H2O 3.1 0.8 –2,840 –9,770 –686 –2,338 DG°′= –14.8 kcal/mole = –61.9 kJ/mole (PEP) 10 Metabolism Phosphates: Ranking by the Standard Free Energy of Hydrolysis Phosphate can be transferred from compounds with higher ΔG°′ to those with lower ΔG°′. Metabolism TABLE 13-5 Total Concentrations of Adenine Nucleotides, Inorganic Phosphate, and Phosphocreatine in Some Cells Concentration (mM)a ATP ADPb AMP Energy Charge Pi Rat hepatocyte 3.38 1.32 0.29 4.8 0 Rat myocyte 8.05 0.93 0.04 0.81 0.94 0.87 0.94 0.86 8.05 28 2.72 4.7 Rat neuron 2.59 0.73 0.06 Human erythrocyte 2.25 0.25 0.02 E. coli cell 7.90 1.04 0.82 PCr 1.65 0 7.9 0 a For erythrocytes the concentrations are those of the cytosol (human erythrocytes lack a nucleus and mitochondria). In the other types of cells the data are for the entire cell contents, although the cytosol and the mitochondria have very different concentrations of ADP. PCr is phosphocreatine, discussed on p. 516. b This value reflects total concentration; the true value for free ADP may be much lower (p. 509). Cellular ATP concentration is usually far above the equilibrium concentration, making ATP a very potent source of chemical energy. 11