bioenergetics intro/chpt 14. catabolism & energy prod’n in cells (fig. 4, p487) glycolysis...
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Bioenergetics
Intro/Chpt 14
Catabolism & energy prod’n
in cells (Fig. 4, p487)
• Glycolysis
• Intermediary metabolism
• ATP production
–Mitochondrial
– Chloroplast
Fig. 4, p.487
Regulatory enzymes
• Rate limiting
• Modulators control +/-
– Allosteric
– Covalently modified
– Combination
• Pathway commitment
Metabolic rxns follow trends
• ~ 50 rxns
– Only 5 major types (REMEMBER?)
• Coupling
• Redox rxns impt
Thermodynamics (again!)
G = H - T S
G - = Exergonic = heat given off
H - = Energy released w/ bonding rxn
S + = Increased entropy (incr’d randomness)
Go’ = Std free energy (pH=7, [H2O]=55 M, [reactant]=1 M, T=25oC) = physio cond’s in cell
Thermodynamics (again!)
• For cellular rxn: a A + b B <= > c C + d D at equilib
– K’eq can be written
– K’eq related to Go’ (Table 14-2)
• Can predict Go’ from Keq and vice/versa (Table 14-3)
In real life
• Not all reactants @ 1 M
– Go back to G
G = Go’ + RT ln ([C]c[D]d/[A]a[B]b)
– Theoretical max energy for rxn
• Actual energy available to system < theoretical
In real life – cont’d
• Not all thermodynamically favorable rxns proceed at measurable speeds
– Enzyme catalysis impt
G relationship to k is inverse and exponential (REMEMBER??)
G stays the same
In sequential reactions• If common reactants, products:
Go’ values are additive
– So thermo’ly unfavorable rxn can be driven by thermo’ly favorable rxn coupled to it
• Keq values are multiplied
– So see large differences in Keq of coupled rxns
• Commonly coupled to endergonic rxns:
– ATP hydrolysis: Go’ = -30.5 kJ/mole
– Coupling hydrol of n ATPs raises Keq by 108n
ATP hydrolysis adds energy
• Products of hydrolysis are resonance stabilized (14-1)
– Decr’d electrostatic repulsions in ADP
– Pi O’s can share – charge
Fig. 14-1
ATP hydrolysis adds energy
• Mg coordinates w/ ADP (14-2)
ATP hydrolysis adds energy
• Pi or AMP often cov’ly couples w/ reactants
High energy intermediate
– Larger G when cleaved
– Glutamate (14-8)
– First step in glycolysis activates glucose
Fig. 14-8
Some notes…
• ATP may bind non-covalently to protein; hydrolysis provides energy for conform’l change
– Ex: Na+/K+ ATPase
• Other phosphorylated cmpds release energy w/ cleavage of Pi (Table 14-6)
– Products also often resonance stabilized (14-3, 14-4)
– BUT original source of Pi is ATP ADP + Pi
Fig. 14-3
Fig. 14-4
Some (more) notes…
• Thioesters impt
– Acetyl CoA example (14-6)
– Greater G for hydrolysis (14-7)
• Nucleoside triphosphates are source of nucleotides inc’d into DNA, RNA (w/ release of energy) (14-12)
Fig. 14-6
Fig. 14-7
Fig. 14-12
Biological Oxidation Reduction Reactions (Redox)
• Flow of e-’s changes redox state of reactants, products
– Reactant that goes from more red’d more ox’d
– e-’s accepted by another molecule, goes from more ox’d more red’d
Redox Rxns – cont’d
• Battery as example of e- flow energy
– Two linked sol’ns w/ differences in affinities for e-
– Coupled through e- carrier
– Carrier associated w/ motor, which can give off energy (in the form of work)
Redox Rxns – cont’d
• Cellular analogy
– Two sol’ns = two molecules w/ differing affinities for e-
– e- carrier = cofactor (molecule)
–Motor = ATP synthesis “machine” in mitochondrion which can give off energy (in the form of a chemical with high potential chemical energy)
Redox Rxns – cont’d
• Metabolism of nutrients converts cmpds from more red’d more ox’d states
– By LEO/GER, nutrient loses electrons (e-‘s)
– e-‘s released to system BUT are NOT free in cytoplasm
– e-‘s transferred to carrier mol’s
• By LEO/GER, carrier mol’s now red’d
Biological Oxidation Reduction Reactions (Redox) – cont’d
• Red’d carrier mol’s bring e-‘s to mitoch
– Electron transport system
– Coupled to oxidative phosph’n
ATP prod’d
Redox Rxn’s (cont’d)
• Rxns of e- flow (reductant [or e- donor] oxidant [or e- acceptor]) can be additive
• Imptc – free energy of system changes w/ change in red’n potential of reactants/products in rxn
E = diff in red’n potentials of reductant, oxidant
– Related to free energy of system ( G) (eq’n 14-6)
– Use to calc G’s for biol. oxn’s
Redox Rxn’s (cont’d)
• e- flow from lower red’n potential higher red’n potential (Table 14-7)
• Eo’ additive if coupled rxns have common intermed’s
– Use to calc G’s for biol. oxn’s
Redox Rxn’s (cont’d)
• Cells’ rxns (incl redox) involve organic cmpds
• Consider “ownership” of e- by C in a cmpd (14-13)
• Ox’n C-cont’ng cmpds often w/ bonding O to C, displacing H
–More red’d cmpds – more H’s, fewer O’s
–More ox’d compds – more O’s, fewer H’s
Fig. 14-13
Redox Rxn’s (cont’d)
• Oxidation may occur in 4 ways
– Electrons transfer directly
– As H+ + e-
– As combination w/ O2
– As :H- (hydride ion)
• Common mechanism w/ carriers
• “Reducing equivalents”
Nicotinamides -- NAD, NADP
• When ox’d: NAD+, when red’d: NADH
• One C on nicotinamide ring accepts e- as :H-
• Hydride donor also releases one H+ to system
– Overall: NAD+ + 2e- + 2H+ NADH + H+
Fig. 14-15a
NAD, NADP – cont’d• NADP+ preferred by some enz’s, species
– [NAD+/NADH] >> [NADP+/NADPH]
• [NAD+] usually > [NADH]
– Commonly donates or accepts hydride?
• [NADP+] usually < [NADPH]
• Enz’s = oxidoreductases or dehdrogenases
– > 200 (Table 14-8)
• Loosely assoc’d w/ deHases
– Move between enzymes
– Recycled by cell
Flavin Nucleotides – FMN, FAD
• Der’d from riboflavin
• Isoalloxazine ring accepts 1 or 2 e-
– Semiquinone (partly red’d)
– Quinone (fully red’d)
• Often bound more tightly to enz’s
– “Prosthetic grps”
• Varied enz’s associate w/ flavins
– Table 14-9
Fig. 14-16
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