QUATERNARY STRUCTURES AND COMPLEX ENZYMES Advantages of complex

QUATERNARY STRUCTURES AND COMPLEX ENZYMES Advantages of complex

QUATERNARY STRUCTURES AND COMPLEX ENZYMES Advantages of complex (quaternary) protein structures Stability:: de cre ase d surface-to-volum e -> m ore hy:drophobic interactions Ne w sit e s: e.gg .g, imm unoglobulin bind ing sit es Couple d reactions: try:ptophan sy:nthetase: Indoleg ly:cerol-P --> G3P +indole indole + ser --> trp purine (A,A,, G) sy:nthesis: 10 re actions, 6 enz y:m es, 1 comp lex (A,inpurine depleted m edium ) Cooperativit y:: e .gg.g, all

ostery: Exam ple ofcooperativity:: Mb/Hb Shows advantage of quaternary: structure Show exam ple offle xibility:: low G of h sape change Myoglobin MW ca 17,00 0 daltons (A,A,MU)) 75% -he lix He m e prosthetic group: protoporphy:rin ring plus Fe2+ He m e binds O 2 as porphy:rin-Fe2+ -O 2 , color change from brown --> red Serve s as an O 2 buffer Hy:perbolic saturation curve shows that there is no coope rative activity: 100%

A/OD (Mb O2 total Mb) P50 50% P50 ~ 1 0% 0 pO2 (torr, mmHg) 100 Why is this an O2 buffer? High slope below the P50 means that considerable Mb is charged (or uncharged) for a small change in pO2 (i.e., as pO2 drops, MbO2 replenishes O2.)

Hemoglobin Tetram er ofm y:oglobin-like subunits, each with.g.g.g He m e prosthetic groups: protoporphy:rin ring plus Fe2+ MW ca 4 x 17,0 00 daltons 75% -he lix Com plexe d with O2 , for m porphy:rin-Fe2+ -O 2 , brown --> red Better O2 buffer (at a higher [O2]) Sigmoid saturation curve shows that there is coordinate activity: O2 is a positive, homotropic, allosteric effector 100 Lung: pH 7.6

P 50 = 26 to rr % HbO2 total Hb 50 0 0 20 30 pO2 (torr) 600 Better O2 buffer (at a higher [O2]) Sigmoid saturation curve shows that there is

coordinate activity: positive, homotropic, allosteric effector Bohr effect: H+, CO2 promote dissociation of O2 from Hb-O2: negative, heterotropic, allosteric effector 2,3-bisPGA also promotes dissociation of O2 Hb in RBCs in tissue O2 H+ CO2 Hb: ~300 x 106/cell O2

in lung CO2 Lung conditions (low H+, CO2) promote O2 saturation; Tissue conditions (high H+, CO2) promote O2 release; The allosteric effects require 2,3-BPG. Allosteric effects match the saturation curve to the conditions in lung and tissue. TBS8-14. The curves below show oxygendissociation from hemoglobin under several conditions. Assume that curve 3 corresponds to hemoglobin with physiological concentrations of CO2 and 2,3-BPG at pH 7. Which curves represent each of the following perturbations? a. Decrease in CO2 b. Increase in 2,3-BPG (What is the effect of depleting 2,3-BPG when blood is stored?)

c. Increase in pH. d. Loss of quaternary structure. 1 2 3 p 4 O 2 Motor Proteins Types Linear:

substrate motor Microtubules (tubulin) dynein (+ to -) Microtubules (tubulin) kinesin (- to +) Microfilaments (actin) myosin DNA

helicases Rotary: Bacterial agell fl a F0 F1 A,TP sy:nthase s Motion dep e nds on Flexible 3 o structure Re versible binding A,TP hy:droly:sis affecting bind ing Microtubule -- kinesin Microtubule: right-handed hollow helix of tubulin / dim ers + Kinesin: left-handed he lix with tw o globular heads

(loose) (tight) ADP ATP 2 1 (hydrolysis) (exchange) H2O Pi ATP ADP ATP 1 ADP

ATP 2 1 ADP 2 1 Rotation ATP 2 Rotation

Each step depends on flexibility (rotation) Each step hydrolyzes one ATP (--> ADP + Pi) Each step involves an exchange reaction (There is another motor protein, dynein, which moves along microtubules. Its 4-A crystal structure was recently reported (Science 331:1159, 3/4/11), and a theory for how it moves along microtubules has been advanced (Science 335:221, 1/13/12.) ) QuickTime and a decompressor are needed to see this picture. Actin-myosin Microfilaments: right-handed double helix of actin monomers G-actin ionic strength F-actin high low

Myosin Left-handed coiled-( -helix) coil: heavy chain NH2 heavy chain COOH light chains ATP ADP-P ADP-P H2O ATP +ATP -ADP -Pi

High-stress conformation Myosin-actin connection form Low-stress conformation: actin moves or myosin moves Myosin is an ATPase. Would you expect the addition of actin to increase or decrease ATP hydrolysis activity? ATP ADP-P

ADP-P H2O ATP +ATP -ADP -Pi High-stress conformation Myosin-actin connection form Low-stress conformation: actin moves or myosin moves

Myosin is an ATPase. Would you expect the addition of actin to increase or decrease ATP hydrolysis activity? (Reaction rate: 0.05 s-1 10 s-1) Actomyosin in muscles A b and I band Contraction: Sliding in the A band from myosin-actin connections Why rigor mortis? (loss of ATP) Conclusion: Flexibility in protein structures allows more complex functions Reversible O and CO binding

Reversible protein-protein (e.g., kinesin-microtubule) binding 2 2 Shows the importance of low G in protein shape changes

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