Chapter 10. Oxygen Transporting Proteins

Transcription

1 Chapter 10 Oxygen Transporting Proteins

2 Oxygen-transport proteins Vertebrates Myoglobin (Muscle) Hemoglobin (Blood) Invertebrates Hemerythrin Hemocyanin Heme Cu Cu O 22 Cu O 2 -binding site of hemocyanin

3 Myoglobin

4 Myoglobin s oxygen-binding curve is hyperbolic Mb + O 2 MbO 2 K= [Mb][O 2 ] [MbO 2 ] Fractonal saturation (YO 2 ) YO 2 = [MbO 2 ] [Mb]+ [MbO 2 ] Because [MbO 2 ] = [Mb][O 2 ] K [Mb][O 2 ] YO 2 = [Mb] + K [Mb][O 2 ] K YO 2 = [O 2 ] K + [O 2 ] YO 2 = po 2 K + po 2

5 Hemoglobin A tetrameric protein with two alpha and two beta chains Both chains fold into a Mb-like structure An oxygen carrier protein Provides oxygen to different types of tissues Hb has two different conformations; oxygen-bound and unbound

6 Deoxyhemoglobin

7 Oxyhemoglobin

8 Differences between deoxy- and oxy-hb

9 Why do we need blood to transport O 2? Why do we need hemoglobin? Why does hemoglobin have 4 subunits?

10 In organisms larger than 2 mm, diffusion is too slow to supply the tissues with O 2 We need hemoglobin to increase O 2 solubility of blood

11 Hemoglobin transports O 2 from the lungs to the tissues Take up as much O 2 in the lungs as possible Leave as much O 2 behind as possible in the tissues How is the affinity to O 2 controlled?

12 Hemoglobin s oxygen-binding curve is sigmoidal 26

13 Sigmoidal binding curve suggests cooperativity Hb contains 4 O 2 binding sites Binding at one site affects the binding at other sites Binding of O 2 to one site increases the affinity of the remaining sites (lungs) Release of O 2 from one site reduces the affinity of the remaining sites (tissues)

14 Mathematical model (Hill equation) Assuming that hemoglobin (Hb) bound n molecules of O 2 in a single step with infinite cooperativity, n (Hill coefficient): Degree of cooperativity n = 1: noncooperative (myoglobin) n > 1: positively cooperative n < 1: negatively cooperative

15 Hill plot Slope = n Intercept = p The last (4 th ) O 2 to bind to hemoglobin does so with 100-fold greater affinity than the first

16 Mechanism of cooperativity (Perutz Mechanism) R State (oxyhb) T State (deoxyhb)

17 Switch between T and R state

18 Stabilization of T state without Fe-O 2 bonds

19 Hemoglobin is the perfect O 2 transporter Sigmoidal O 2 binding curve Decrease in ph reduces the affinity for O 2 CO 2 reduces the affinity for O 2 Interaction between 4 subunits

20 Bohr effect Decrease in ph reduces the affinity for O 2 Protonation of Hb at low ph N-amino group (α subunits) C-terminal His (β subunits) When protonated these groups are involved in ion-pairs that stabilize the T form (deoxy-hb) HbH + + O 2 HbO 2 + H +

21 Stabilization of T state without Fe-O 2 bonds

22 The O 2 affinity increase with increasing ph

23 Hemoglobin and CO2

24 CO 2 can also bind directly to Hb N-terminal NH 2 groups reacts with CO 2 to form a carbamate R-NH 2 + CO 2 R-NH-COO - + H + The T form binds more CO 2 as carbamate The carbamate formation stabilizes the T form (O 2 release)

25 H + production in tissues CO 2 + H 2 O H + + HCO 3 - Lactic acid H + + Lactate -

26 O 2 H + CO 2

27 Lung High ph Low pco 2 High po 2 O 2 binding Tissue Low ph High pco 2 Low po 2 O 2 release

28 BPG decreases the affinity for O 2 Synthesized in red blood cells Lowers the affinity for O 2 In the absence of BPG, the affinity for O 2 would be too high Binds to the central cavity of T-state Hb

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30 BPG plays a role in altitude adaptation and delivery of O 2 to the fetus At high altitude the po 2 is lower Increase in BPG concentration Small decrease in O 2 -binding (lungs) Much larger increase in O 2 release (tissues)

31 Allosteric effect Binding of a molecule to one site affects the activity ofthe protein at a different site Positive effector (modulator) Negative effector (modulator) O 2 binding at one site increases the affinity of the other sites for O 2 H + decreases the affinity of Hb for O 2 CO 2 decreases the affinity of Hb for O 2 DPG decreases the affinity of Hb for O 2

32 Allosteric models Symmetry model Sequential model (induced-fit model)

33 Symmetry model of allosterism

34 Sequential model of allosterism

35 Abnormal hemoglobins

36 Normal vs Sickled blood cells

37 HbS Fiber

38 HbS fiber

39 Malaria and HbS The sickle cell gene confers resistance to malaria Malaria parasite increases the acidity of the infected erythrocytes Promote formation of deoxyhb Promote sickling of the infected erythrocytes Removal of the infected sickle cells by the spleen (early stage) Disruption of the parasite by sicking (late stage)