BIOCHEM 2BB3 Chapter Notes - Chapter 5: Myosin

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2BB3- Chapter 5: Protein Function
Myoglobin and Hemoglobin
-Protein can be shaped as globular proteins or fibrous networks
oCollagen, structural (extracellular matrix) protein, help form the cytoskeleton
oMotor proteins control movement of organelles within the cell
-Myoglobin: small; lacks beta structure, and 121/153 AA are found in 8 alpha helices, labelled A—H
(right picture)
-Hemoglobin: tetrameric: 2 alpha and 2 beta chains; each subunit (globin) looks like myoglobin;
Heme:
-Both: heme found in hydrophobic pocket between E and F helices
oHis F8 ligands the F(II) ion, and His E7 that H-bonds with O2
Heme:
- Heme is a prosthetic group (constituent that allows protein to carry out a function) = binds to O2
-Central Fe2+ binds to 4 N ligands, can bind to a His and reversibly bind to O2 = carrying abilities
oHowever, alone, not an effective O2 carrier as the Fe2+ can be easily oxidized Fe3+, and
can’t bind to O2
[O2] Binding
Mb + O2
MbO2, K = [Mb][O2] / [MbO2]
Fractional Saturation,Y: portion of total myoglobin molecules bound to O2
- pO2, partial pressure of oxygen
Y = pO2 / (K + [O2])
-As O2 increases, more O2 begins to bind to the same myoglobin until it is saturated
- p50: oxygen pressure of 50% saturation; when myoglobin’s Y half-saturated, and [O2] = K
Related by Evolution
Common ancestor myoglobin and monomeric hemoglobin hemoglobin with alpha and beta chains…
-The AA sequences of 3 globin proteins are only 18% identical
-The tertiary structure of many proteins, although have different AA sequences, can have similar
structures
-The globins are homologous proteins from a common ancestor
oInvariant residues- necessary for protein’s structure and function
oConservatively substituted and have less selective pressure
oVariable- substituted for many AA- not necessary for protein’s structure and function
O2 Binds Cooperatively to Hemoglobin
- Hematocrit: way to measure oxygen-carrying capacity of
blood; 40% in women and 45% in men
-Myoglobin: pO2 vs. Y is hyperbolic
oHalf-saturated at 2.8 torr
oOnce one O2 binds, more O2 begins to bind
-Hemoglobin: pO2 vs. Y is sigmoidal
oHalf-saturated at 26 torr
oOnce one O2 binds, hemoglobin is reluctant at
first but when pO2 increases (deoxy oxy conformation), O2 binds sharply until saturation
Reverse: Unwilling to give up O2 at first but when pO2 decreases, O2 released easily
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-Hemoglobin’s four subunits aren’t independent but work together in a united fashion =
cooperative
Conformational Shift:
-Deoxyhemoglobin, T state
oFe has 5 ligands => porphyrin ring is domed shape with Fe slightly out
oWhen Fe is out, a protein is in the T state = unfavourable O2 binding
-Oxyhemoglobin, R state
oFe has 6 ligands
oAfter O2 binds, Fe is moved to the centre and the F helix is moved up
-Hemoglobin is an allosteric protein- has multiple binding sites- that allows small molecules
(ligands) to bind to it and alter the ligand-binding affinity of the other sites
Bohr Effect and Bisphosphoglycerate
-Switching between T and R states alters the microenvironments of ionisable groups un the protein,
like: N-terminal amino groups of alpha subunits and the two His residues near the beta subunits
becomes acidic when O2 binds
- Bohr Effect: as the pH decreases, hemoglobin’s O2-binding affinity is reduced
oTissues absorb O2 and release CO2 into RBC
oH+ released, increases acidity & the high [O2,] in the lungs promotes O2 to be picked up by
hemoglobin
BPG: 2,3-Bisphosphoglycerate
-The presence of BPG stabilizes the deoxy conformation of hemoglobin
-The –ve charges in BPG interacts with the +ve charges in the deoxy conformation
-Without BPG, hemoglobin has a high O2 affinity and would bind to O2 tightly (even in low p O2)
-Fetal hemoglobin lacks BPG interaction and so, has a higher O2 affinity than adult hemoglobin,
which helps transfer O2 from the maternal blood stream to the placenta
Structural Cytoskeletal Proteins
1. Microfilaments made up of actin filaments
2. Intermediate filaments made up of coiled coils
3. Microtubules made up of tubulins
Microfilaments
-Support plasma membrane, determines cell shape and generate cell movement
- F-actin: polymerized actin, and G-actin: filamentous actin
-The end with ATP-binding is the –ve end and the opposite is +ve end
-Actin polymerization, activated by ATP, is slow (actin dimers and trimmers are unstable) but once
a long polymer is formed, actin can be added at both ends (+ve end is faster)
-Catalyzed by F-actin, most actin subunits in microfilaments are bound to ADP
Actin Filaments:
-Actin polymerization is reversible and dynamic
- Treadmilling: net rate of actin addition = net rate of actin removal
-Formation of polymers is favoured
-Polymerization can cease on end with a capping proteins
oSome proteins interfere with the actin-actin interaction, causing it to break unless the end
was capped
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