Protein Function I (hemoglobin)

Question 1

What are the three post translation modifications of proteins?

Answer 1

phosphorylation, glycosylation, proteolysis

Question 2

what is phosphorylation?

Answer 2

transfer phosphate from ATP to ser, thr, tyr
catalyzed by protein kinases removed by protein phosphatases
regulation of metabolism and signal transduction

Question 3

what is glycosylation?

Answer 3

attachment of complex carbohydrate groups to asn and ser
secreted extracellular or lumenal proteins
affects protein stability, targeting, recognition

Question 4

what is proteolysis?

Answer 4

enzymatic cleavage of polypeptide chain at specific sites
activation of other proteases
synthesis of peptide hormones

Question 5

what is the function of ubiquitin and proteasomes in PTM?

Answer 5

ubiquitin targets a protein with polyubiquitin chain and takes it into proteasome the polyubiquitin chain then leaves and protein is destroyed

Question 6

What is myoglobin?

Answer 6

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

Question 7

What helps prevent the oxidation of Fe2+ to Fe3+?

Answer 7

burying the heme in a hydrophobic pocket

Question 8

what is the Fe2+ coordinated by?

Answer 8

four nitrogen atoms of the heme ring, His F8 and by O2 bound to His E7

Question 9

what shape in the oxygen binding to myoglobin?

Answer 9

hyperbolic

Question 10

what is the fractional saturation (Y) of myoglobin?

Answer 10

Y= [MbO2]/[Mb]+[MbO2] or Y=pO2/K + pO2

Question 11

when does Y = 0.5?

Answer 11

when pO2=K

Question 12

true or false myoglobin and hemoglobin are homologous?

Answer 12

TRUE

Question 13

what does hemoglobin do?

Answer 13

carries O2 from the lungs to the tissues

it is a tetrameric protein (alpha2beta2)

Question 14

Do Mb and Hb molecules have similar tertiary structure?

Answer 14

YES only 18% sequence identity tho so thats kinda whack

the invariant residues signify critical function while sequence differences indicate evolutionary divergence

Question 15

what is the O2 dissociation curve shape of Hb?

Answer 15

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

Question 16

what are the two Hb states and their O2 binding affinities?

Answer 16

1. Taut –low affinity for O2

2. Relaxed –high affinity for O2

Question 17

What happens if O2 is bound to one of the Hb subunits in the T state?

Answer 17

moves helices E and F relative to each other; transmits a configuration change to other subunits

Question 18

what is the bohr effect?

Answer 18

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.

Question 19

what is the affect of CO2 on Hb?

Answer 19

it reacts directly with N terminus amino groups to form carbamate (stabilizes T)
some CO2 is transported to the lungs this way

Question 20

what is the affect of BPG?

Answer 20

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

Question 21

what is the molecular basis of sickle cell anemia?

Answer 21

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

Question 22

If someone was a carrier for the Hbs mutant would they be affected by the trait at all?

Answer 22

they would be moderately affected under some conditions