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