Exam 2 Study Guide - Protein Function Flashcards
What is a heme?
protein-bound prosthetic group (present in myoglobin and hemoglobin)
What is the structure of heme?
complex organic ring structure, protoporphyrin, with a bound Fe (2+) atom
Describe the binding of Fe to the heme and globin molecules.
- Fe bind to heme via 6 bonds (4 to nitrogen atoms in the porphyrin ring and 2 perpendicular to the porphyrin)
- Perpendicular bonds: one is occupied by proximal His residue and one is the binding site for molecular oxygen…Fe (2+) binds O2 reversibly…Fe (3+) does not bind O2
What is Ka – the association constant?
Provides a measure of the affinity of the ligand L for the protein (higher Ka, higher affinity)
Define “Y” and understand the relationship between Y and L.
The amount of ligand present is proportional to the ratio between bound ligand and free protein (hyperbolic function)
What is Kd – the dissociation constant.
Reciprocal of Ka (lower Kd, higher affinity)
Proteins A and B both bind ligand L. Which one has the higher affinity for the ligand?
Protein A has the higher affinity for the ligand because Kd is lower at ½ of the available ligand, resulting in a higher affinity
Explain how myoglobin increases the binding affinity of Fe for oxygen.
Myoglobin binding to heme causes the affinity for O2 to increase; distal His rotates to open and close the pocket
Describe how the binding of oxygen changes the three-dimensional structure of hemoglobin.
Hydrogen bond between the side chain of His E7 and bound O2 electrostatically stabilizes the Fe-O2 polar complex (20,000-fold stronger binding affinity of free heme for CO compared with O2 declines to 40-fold)
Describe the composition of muscle proteins.
Composed of actin (thin) and myosin (thick) protein; filaments that interact and slide past each other to bring about contraction
Describe how actin and myosin interact to facilitate muscle contraction.
The myosin thick filaments and actin thin filaments are sliding past one another, making the muscles contract/stiffen as rigor mortis sets in. To be more specific, there are 4 steps involved in the molecular mechanism of muscle contraction. First, ATP binds to the myosin head, causing myosin to dissociate from actin. Then, the bound ATP is hydrolyzed and a conformational change in myosin occurs (ADP + P are still attached to myosin). Third, myosin heads attach to actin filaments, causing release of phosphorus…attachment occurs upstream of where it was previously attached. Lastly, the release of P triggers a “power stroke” conformational change in myosin, causing it to slide against actin filament (ADP is released).
What is hemoglobin?
responsible for O2 transportation in the bloodstream
What is myoglobin?
facilitates O2 diffusion in muscle tissue
Which part of the hemoglobin molecule binds to oxygen:
Heme
How many bonds can the iron in hemoglobin make?
6