Chapter 5 - Protein Function

main types of proteins

globular proteins

structural proteins

motor proteins

enzymes

structure of myoglobin

no beta sheets, 8 alpha helices

hemoglobin structure

tetrameric protein with four subunits that each resemble myoglobin

a fully functional myoglobin molecule contains…

polypeptide chain plus iron-containing porphyrin derivative heme

heme is…

a prosthetic group: organic compound that allows a protein to carry out some functions that the polypeptide alone cannot perform

generally what myoglobin does

transports O2 via the Fe in heme

equation to describe binding of myoglobin to O2

Mb + O2 → / ← MbO2

dissociation constant of myoglobin/O2 binding

K = ([Mb][O2])/[MbO2]

fractional saturation variable and definition

Y, the proportion of myoglobin molecules that have bound O2

calculation of fractional saturation

Y = (bound Mb)/(total Mb) → [MbO2]/([Mb] + [MbO2]) → pO2/(K+pO2)

how is O2 concentration expressed in Y?

as pO2 → partial pressure of oxygen, since it’s a gas, in torr

y = pO2/(K+pO2)

Y v. pO2 plot for myoglobin

hyperbola

myoglobin is said to be saturated with oxygen when…

virtually all myoglobin molecules have bound O2

what is K/p50

oxygen concentration/pressure at which myoglobin is half-saturated (half-maximal [O2], pO2 at 50% saturation)

hemoglobin is a…

heterotetramer

structural elements of all hemoglobin molecules

heme group in a hydrophobic pocket, His F8 that ligands the Fe(II) ion, His E7 that forms a hydrogen bond to O2

how similar are myoglobin and hemoglobin and what does this indicate?

~18% identical sequences between Mb, Hb → indicate a common evolutionary origin

invariant residues & purpose

identical residues in all globins (including Mb and Hb) → essential for structure and/or function of proteins and cannot be replaced by other residues

hematocrit definition

percentage of the blood volume occupied by red blood cells

hematocrit used to determine

oxygen-carrying capacity of the blood

hematocrit normal range in humans

40% in women - 45% in men

anemia

having too few red blood cells

relationship between Y and pO2 for hemoglobin chart

sigmoidal (solid line)

p50 for myoglobin

2.8 torr

p50 for hemoglobin

26 torr

which line is which

solid → Hb

dashed → Mb

why is the O2 in Hb lungs → Mb in muscles oxygen-delivery system efficient?

the tissue pO2 corresponds to the part of hemoglobin binding curve where the O2 affinity falls off most sharply

relative affinity to oxygen of Mb, Hb

Mb: high

Hb: low

myoglobin general function in oxygen delivery system

relays oxygen from red blood cells to tissues (muscle cells)

hemoglobin general function in oxygen delivery system

transfers oxygen from lung cells to muscle cells

two conformations of hemoglobin

oxy and deoxy

shift between oxy and deoxy primarily involves…

rotation of one alphabeta unit relative to the other

T state Hb

corresponds to deoxyhemoglobin

R state Hb

corresponds to oxyhemoglobin

how to switch between T and R conformation?

First oxygen binding switches from T → R, first oxygen releasing switches from R → T

hemoglobins and other proteins with multiple binding sites are known as…

allosteric proteins

what happens in allosteric proteins

binding of small molecule (ligand) to one site alters the ligand-binding affinity of the other sites

two factors affecting oxygen binding

O2 pressure, proton concentration, BPG

how does conformational change from deoxy → oxy decrease hemoglobin’s O-binding affinity

several groups in the protein are altered and become more acidic, releasing H+ when O2 binds to the protein → decreased pH = less oxygen affinity

Bohr effect

the reduction of hemoglobin’s oxygen-binding affinity when the pH decreases

How does Bohr effect generally work/balance out? description and picture

Hemoglobin picks up O2 in the lungs. In the tissues, H+ derived from the metabolic production of CO2decreases hemoglobin's affinity for O2, thereby promoting O2 release to the tissues. Back in the lungs, hemoglobin binds more O2, releasing the protons, which recombine with bicarbonate to re-form CO2.

equation of Bohr effect, oxygen transport

BPG is…

an additional mechanism to fine-tune hemoglobin function; is 2,3-biphosphoglycerate

what does BPG look like

how/when does BPG bind

binds in the central cavity of hemoglobin, but only in the T (deoxy) state

how does BPG presence help hemoglobin function

BPG presence stabilizes the deoxy conformation of hemoglobin; without BPG, hemoglobin would bind O2 too tightly to release it to cells

BPG’s effect on hemoglobin

reduces hemoglobin’s affinity for O2 by stablizing the deoxy conformation

why does fetus hemoglobin have a higher affinity to oxygen than adult hemoglobin?

fetus’ hemoglobin can’t bind BPG so affinity remains high in deoxy configuration

abbr for regular and sickle cell hemoglobin

regular: hemoglobin A

sickle cell: hemoglobin S, Hb S

who develops sickle cell disease, common populations

people with two copies of defective gene, predominantly affects population of African descent

what makes sickle cell hemoglobin the wrong elongated shape

In normal hemoglobin, the switch from the oxy to the deoxy conformation exposes a hydrophobic patch on the protein surface between the E and F helices.

In sickle cell hemoglobin, the hydrophobic valine residues on hemoglobin S are optimally positioned to bind to this patch. This intermolecular association leads to the rapid aggregation of hemoglobin S molecules to form long, rigid fibers

what happens in heterozygotes of hemoglobin S

only about 2% of red blood cells undergo sickling → breaking these sickled cells releases Hb with toxic free heme groups, so body increases heme oxygenase production to degrade them → this produces carbon monoxide, which tells cells to decrease response to inflamatory signals

overall → less severe inflammation and tissue damage that would normally occur during plasmodium infection

what does mutant hemoglobin C do

makes red blood cells a bit more rigid than normal, leads to mild anemia

thalessemias are cuased by…

genetic defects that reduce rate of synthesis of alpha or beta globin chains