Protein Function I (hemoglobin) Flashcards
What are the three post translation modifications of proteins?
phosphorylation, glycosylation, proteolysis
what is phosphorylation?
transfer phosphate from ATP to ser, thr, tyr
catalyzed by protein kinases removed by protein phosphatases
regulation of metabolism and signal transduction
what is glycosylation?
attachment of complex carbohydrate groups to asn and ser
secreted extracellular or lumenal proteins
affects protein stability, targeting, recognition
what is proteolysis?
enzymatic cleavage of polypeptide chain at specific sites
activation of other proteases
synthesis of peptide hormones
what is the function of ubiquitin and proteasomes in PTM?
ubiquitin targets a protein with polyubiquitin chain and takes it into proteasome the polyubiquitin chain then leaves and protein is destroyed
What is myoglobin?
monomeric O2 carrier in muscle cells has 8 alpha helices
O2 is bond to Fe2+ group in a porphyrin ring (heme) prosthetic group (something attached to a protein that helps with function), wedged between helices E & F
What helps prevent the oxidation of Fe2+ to Fe3+?
burying the heme in a hydrophobic pocket
what is the Fe2+ coordinated by?
four nitrogen atoms of the heme ring, His F8 and by O2 bound to His E7
what shape in the oxygen binding to myoglobin?
hyperbolic
what is the fractional saturation (Y) of myoglobin?
Y= [MbO2]/[Mb]+[MbO2] or Y=pO2/K + pO2
when does Y = 0.5?
when pO2=K
true or false myoglobin and hemoglobin are homologous?
TRUE
what does hemoglobin do?
carries O2 from the lungs to the tissues
it is a tetrameric protein (alpha2beta2)
Do Mb and Hb molecules have similar tertiary structure?
YES only 18% sequence identity tho so thats kinda whack
the invariant residues signify critical function while sequence differences indicate evolutionary divergence
what is the O2 dissociation curve shape of Hb?
sigmoidal which allows Hb to release much more O2 than Mb at lower pO2 of tissues. this behaviour is typical of allosteric multi subunit proteins
what are the two Hb states and their O2 binding affinities?
- Taut –low affinity for O2
2. Relaxed –high affinity for O2
What happens if O2 is bound to one of the Hb subunits in the T state?
moves helices E and F relative to each other; transmits a configuration change to other subunits
what is the bohr effect?
an allosteric effector which changes the pH to being more acidic because of CO2 produced in tissues forming H2CO3 which dissociates to produce H+. The decreased pH protonates key histidine side chains and alpha beta interface which overall stabilizes the T state.
what is the affect of CO2 on Hb?
it reacts directly with N terminus amino groups to form carbamate (stabilizes T)
some CO2 is transported to the lungs this way
what is the affect of BPG?
it is negatively charged regulator molecule the binds to the charged hole at center of HB only in the T state which stabilizes it
BPG is increased at high altitudes and in chronic anemia which favours O2 release
drecreased in transfused blood
what is the molecular basis of sickle cell anemia?
a point mutation in the beta subunit at the surface–changes glue which is polar to val which is hydrophobic
in the deoxygenated state a hydrophobic pocket is created where the Hb can stick together which changes the biconcave shape to sickle
If someone was a carrier for the Hbs mutant would they be affected by the trait at all?
they would be moderately affected under some conditions
