MCB 2000 Exam 2

*diffraction through a protein
-"maps out" coordinates of every atom in the molecule to determine structure
-need pure protein to form crystals for good data

*Evolutionary related; share sequence homology
*Both contain heme prosthetic group

*a way to incorporate an atom that wouldn't normally attach to an amino acid

*Stores Oxygen in muscle
*single polypeptide chain; monomer
*similar numbers of A.A's in sequence

*Transports Oxygen, CO2, and H+
*Important role in pH balance
*Tetramer: 2 alpha subunits 141 residues, 2 beta subunits of 146 residues
-able to bind 4 oxygens
-releases O2 under low O2 tension, takes away CO2 and H+ ion

*Oxygen dissolves poorly in water
-diffusion thru tissue is ineffective
*Transition metals have strong tendency to bind oxygen
-very reactive in free form, esp iron
*Iron must be sequestered to render it less reactive

*Protoporphyrin ring
*Hydrophobic, planar
*Nitrogens prevent Fe2+ to Fe3+ through their e- donating capacity (Fe2+ binds oxygen reversibly, Fe 3+ does not)
*heme is sequestered in protein's structure

*Hb must bind oxygen in lungs and release in capillaries
*When first oxygen binds to Fe in Heme, Fe is drawn into protoporphyrin ring
-disrupts noncovalent interactions that changes confirmatio
*02 has more structural effects on hemoglobin than myoglobin

*As O2 binds to Fe, F-helix changes
*loss of interaction between F and H helices
-salt bridge between ASP94 and HIS146 is broken
-salt bridge between LYS40 and HIS146 is disrupted
- H bond between VAL98 and TYR145 is disrupted
*Salt bridges = electrostatic interactions

* Shifts greatly in size (decrease) when O2 binds due to 15 degree shift

*Quaternary structure
*Cooperativity leads to a change in function
*Sigmoidal binding curve
- "s" shaped

*"taught", less active state
-T state for Hgb is deoxyhemoglobin
-stable, but different confirmation
-low affinity for O2, promotes release of O2
*Once O2 is binded, 3 noncovalent interactions interupted
-2 salt bridges, 1 H-bond
- R state favored
*what happens in one Hgb subunit will effect the others

*bind in central cavity
-2,3 BPG is negative (3 carboxyl groups)
-interacts with R groups with basic amino acids (NH3+) of lysine and histidine
-electrostatic interactions
*produced during glycolysis in RBCs
-without 2,3 BPG, O2 is binded more "taughtly"
-stabilizes T-state
-very negatively charged

*a more acidic environment favors T state
*H+ and CO2 work together to promote O2 release
*

*Protons react with Histamine side chains. His-146 important in beta subunit; protonation promotes release of O2