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BIOCHEM > CH 4 (L10) > Flashcards

CH 4 (L10) Flashcards

1
Q

Myoglobin

A

(Mb), Myo refers to muscle, exists as a monomer in muscle of reptiles, birds, and mammals, found in muscle

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2
Q

Hemoglobin

A

(Hb), hemo, heme refers to blood, exists as a tetrameric complex
- heme consists of a tetrapyrrole ring system connected by methene bridges (-CH=)

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3
Q

red blood

A

red color associated with blood due to the heme prosthetic group: protein-bound organic molecule essential for the activity of the protein
–> ferrous iron is bound and forms a complex with 6 ligands, 4 being nitrogen atoms of the protoporphyrin IX

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4
Q

sperm whale oxymyoglobin

A

myoglobin is part of a family proteins called globins
–> myoglobin consists of 8 alpha helices in a bundle –> all-alpha helical bundle –> they are angled to allow the R-groups to interdigitate

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5
Q

Myoglobin (Mb)

A

interior is almost all hydrophobic amino acid residues

  • -> the surface is made of both hydrophilic and hydrophobic residues
  • –> the heme prosthetic group occupies a hydrophobic cleft stabilized by weak interactions
  • –> accessibility to oxygen depends on nearby amino acid side chains
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6
Q

Neuroglobin (Ngb)

A

another globin, recently discovered

  • -> preferentially expressed in nervous system tissue
  • -> monomer, 150 aa in length, 16kDA, typical globin fold, binds O2, CO, or NO
  • provides neuroprotection
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7
Q

Cytoglobin (Cygb)

A

another recently discovered Globin
–> expressed widely in tissues, can bind O2, may be involved in protection during oxidative stress, 1900aa in length (human Cygb)

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8
Q

Erythrocytes

A

erythro-red, cytes- cells
oxygen and hemoglobin carrying the cells of the blood in vertebrates,
each 6-8 um in diameter cell carries ~ 3x10^8 molecules of Hb

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9
Q

Hemoglobin (Hb)

A
  • an (α2β2) alpha2beta2 tetramer made of α-globin (7 α helices) and β-globin (8 α helices)
  • each globin is similar to myoglobin in structure
  • each of the four contains a heme prosthetic group
  • 1 α chain interacts extensively with a β chain so sometimes Hb is referred to as an αβ dimer
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10
Q

oxygenation

A

refers to the reversible binding of the oxygen to the heme group

  • -> without oxygen , Hb is called deoxyhemoglobin
  • -> with oxygen, it is called oxyhemoglobin
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11
Q

tissue distribution of vertebrate globins

A

blood - hemoglobin
muscle - myoglobin
neurons - neuroglobin
all organs - cytoglobin

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12
Q

Hb

A

hydrophobic residues allow heme prosthetic group to be buried inside the hydrophobic cleft

  • -> 4 nitrogen help coordinate the iron Fe II
  • -> the fifth ligand is an imidazole ring (His-93) called the proximal Histidine
  • -> the 6th ligand is molecular oxygen (coordinated to the Fe II and imidazole side-chain of His-64
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13
Q

Holding in Place

A

free heme (in solution) does not reversibly bind oxygen in aqueous solution

  • -> Fe2+ is partially oxidized to Fe3+ when oxygne is bound
  • -> an electron is temporarily transferred toward the oxygen atom attached to the iron (dioxygen is partially reduced)
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14
Q

Reversible binding of oxygen

A

binding of oxygen depends on

  • -> concentration of protein
  • -> concentration of oxygen
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15
Q

Fractional saturation (Y) of myo/Hb

A

measured as the amount of which has molecular oxygen bound divided by total (bound + unbound)

Formula:
Y = [MbO2] / [MbO2] + [Mb]

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16
Q

Oxygen-binding curves

A

the curve for myoglobin is hyperbolic (so just one equilibrium constant for binding)
- the curve for hemoglobin is sigmoidal (S-shaped) indicating more than one molecule is binding –> 4 molecular oxygen binds –> these represent environmental pO2 levels

  • -> oxygen binds in a positive-cooperative fashion (1 bound will help the 2nd bind which will help the 3rd)
  • -> physiologically at 3 this stops as only 1 slot is open per Hb molecule in vivo
  • -> notice the pressure at half- saturation (P50)
17
Q

Partial pressure

A
  • is a measurement of the percentage of a give constituent in the gas according to the total pressure
    1 atm - 760 mmHg
  • the partial pressure of oxygen (~21% of the atmospheric gases) at 1 atm = 760 x 0.21 = ~160 mm Hg
18
Q

The cooperative binding wiggle

A
  • when the heme iron (red) of hemoglobin is oxygenated, the proximal histidine residue is pulled towards the porphyrin plane
  • this disrupts ion pairs that cross-link deoxyHb helping cooperative binding to occur
19
Q

conformation changes

A
  • explain the sigmoidal curve of Hb
  • T conformation (tense) resists binding oxygen (low linear line)
  • R conformation (relaxed) facilitates binding of oxygen (high curve line)
  • -> hemoglobin observed in the middle
  • R and T states are in dynamic equilibrium
20
Q

allosteric action

A

binding of oxygen to Hb is regulated by allosteric interactions

21
Q

allosteric modulator or allosteric effector

A

binds to a protein and modulates its activity

22
Q

allosteric protein

A

protein regulated by allosteric effectors

–> substrate/ligand binds when the protein is in R state

23
Q

Allosteric inhibitors

A

bind their own regulatory sites and bind most avidly to the T state

24
Q

Allosteric modulation

A

carried out by conformational changes

25
Q

Allosteric proteins

A
  • are in equilibrium when their R states and T states are rapidly interconverting
26
Q

2,3-biphosphate-D-glycerate

A
  • 2,3BPG is an allosteric effector of Hb in mammals
  • When 2,3BPG is present, the P50 for binding of O2 in Hb goes from 12 to 26 torr
  • 2,3BPG binds to deoxyHB and stabilizes the T state
  • What does that mean for binding to oxygen? Lower affinity
  • In the R state, the binding pocket is too small for 2,3BPG
  • O2 increases affinity
  • 2,3BPG decreases affinity
  • -> Without 2,3BPG, Hb would not release oxygen at the low levels found in tissues
  • -> 2,3BPG allows oxygen to follow its concentration gradient into tissues where it is needed
27
Q

Enter CO2

A
  • -> OxyHb binding is further regulated by the buffering system in the blood
  • -> CO2 decreases the affinity of Hb for O2 by lower pH inside red blood cells
  • -> CO2 is hydrated in RBCs to produce carbonic acid which dissociates to form bicarbonate and a hydronium ion

Formula: CO2 +H20 H2CO3 H+ + HCO3-

28
Q

Changes in pH

A
  • Lower pH in the RBCs leads to a change in the environment
  • -> There are amino acids with ionizable side chains… so what do you think can happen?
  • -> changes in protonation to several groups in Hb (alpha-amino groups with pKa values around 7-8)
29
Q

Carbamate Adducts

A
  • most CO2 is carried in the blood as bicarbonate ions

- -> some is carried by Hb when CO2 reacts with the N-terminus of Hb to form carbamate adducts

30
Q

The Bohr Effect

A

These stabilize the deoxyHb conformation.

  • -> P50 of Hb rises: this is called the Bohr effect
  • -> What happens to the oxygen that was on the Hb? it gets unloaded more easily
31
Q

Oxygen-hemoglobin Dissociation: Exercise

A

you exercise heavily –> your mitochondria churn in your tissues –> your muscles use of a lot of oxygen –> you produce more CO2 which needs to be unloaded in lungs –> CO2 causes decrease in pH in RBCs –> Hb-O2 affinity decreases

Net effect: more CO2 is unloaded, O2 more easily gets into tissues

32
Q

Bohr effect

A

low pH (more acidic), high temp, high P of CO2, high 2,3-BPG

33
Q

Immunoglobulins

A

antibodies (Ig) are part of the vertebrate immune system

  • -> highly specialized glycoproteins
  • -> they are highly adapted to binding to portions of foreign molecules to help eliminate them (antigens)
  • -> they are produced by B-lymphocytes (B cells)

–> IgG: most abundant form of immunoglobulin in your blood

34
Q

IgG

A

most abundant form of immunoglobulin in your blood

  • 2 heavy chains
  • 2 light chains
35
Q

immuglobulin fold

A
  • most common motif for the antibody is the Ig Fold domain (110 residues)
  • 2 antiparallel beta sheet sandwich
  • the variable region of an antibody is so specific, no water molecules are thought to exist between the antibody and the antigen it binds
36
Q

binding antigen

A
  • hydrogen bonds and electrostatic charges stabilize the binding

BIOCHEM (8 decks)

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