Biochemistry Lecture Notes Chapter 7

Front 1

Regulatory Strategies for Enzymes

Back 1

A. Allosteric: distinct regulatory sites
B. Multiple forms of an enzyme
C. Reversible covalent modification
D. Proteolytic cleavage
E. Controlling the amount of enzyme present

Front 2

How much more energy can our cells extract from glucose with oxygen compared to without oxygen?

Back 2

In the presence of O2, cells can extract 15X more energy from glucose than when oxygen is absent

Front 3

How do cells get adequate oxygen?

Back 3

1. Circulatory System
2. Oxygen binding proteins
-Myoglobin: oxygen storage (MUSCLE)
-Hemoglobin: oxygen transport

Front 4

Myoglobin - what is it? what is its function?

Back 4

-a protein expressed in muscle cells
-SINGLE polypeptide chain = MONOMER
-reservoir supply of oxygen when needed

Front 5

Hemoglobin: what is it? function?

Back 5

-protein in RBCs
-Tetramer: 2alpha, 2beta chains
-carries oxygen from lungs to tissues
-transports CO2 and H+ from tissues to lungs
-chains bind O2 COOPERATIVELY

Front 6

Heme

Back 6

-gives red color of blood and muscle
-allows O2 to bind

Front 7

What state is iron in myoglobin/hemoglobin?

Back 7

-Ferrous: Fe2+

Front 8

Structural changes at iron sites in myoglobin and hemoglobin

Back 8

1) Fe2+ binds to 4 pyrrole nitrogen atoms at Heme center
2) 5th coordination site: Fe2+ binds to the imidazole ring of proximal His
3) 6th coordination site: Fe2+ binds O2, changes electronic structure, allows Fe2+ to sit within the plane of the porphyrin in heme

-changes magnetic properties, can take advantage of the magnetic properties of the brain and use fMRI! study brain function

Front 9

Reactive Oxygen Species

Back 9

-myoglobin's structure inhibits their release
-resonance structures distribute electrons between oxygen and iron
-2nd His in O2 binding pocket, donates H bond to O2 --> O2's superoxide ion hcaracter strengthens the bond

Front 10

Myoglobin O2 binding

Back 10

-non-cooperative (simple chemical equilibrium)
-P50 = 2 torr

Front 11

Hemoglobin O2 binding

Back 11

-COOPERATIVE, sigmoid binding, S shaped, lag before binding occurs
-P50 = 26 torr

Front 12

Allosteric enzymes

Back 12

-ADJUST TO MEET THE IMMEDIATE NEEDS OF THE CELL, KEY REGULATOR OF METABOLIC PATHWAYS
-multiple active sites
-sigmoid kinetics
-can have cooperative binding
-regulatory molecules bind to sites other than the active site

Front 13

PO2 tissues

Back 13

20 torr

Front 14

PO2 lungs

Back 14

100 torr

Front 15

hemoglobin cooperativity

Back 15

-allows delivery of 1.7X more O2 than it would if the sites were independent
-66% drop in saturation (38% if independent, 7% for myoglobin)

Front 16

deoxyhemoglobin

Back 16

-T state
-heme groups are well separated
-favors non-binding of the substrate

Front 17

oxyhemoglobin

Back 17

-R State
-1 pair of alpha/beta subunits rotates 15 degrees with respect to the other upon O2 binding
-favors binding of substrate

Front 18

O2 binding: changes hemoglobin structure

Back 18

-dimer rearrangement at interface
-communication between subunits enables cooperativity
-beta chain movement!
-T->R shift allows the binding of O2 to increase the binding affinity at the other sites!

Front 19

Sequential Model

Back 19

-As you bind O2 at one site, you increase the binding affinity of neighboring sites
-One site changes from T to R at a time
-NOT a full conversion from T to R from one binding

Front 20

Concerted model

Back 20

-the overall moecule can only exist in two forms: T and R
-binding O2 shifts the equilibrium between the two states
-when 2/4 are bound, equal likelihood of R and T

Front 21

Actual cooperative binding of O2 to hemoglobin

Back 21

-Sequential when 1/4 occupied
-remains in T state
-binds O2 3X stronger than when fully de-O2
-Concerted when 3/4 occupied
-exists in R state
-binding of 4th O2 --> 20X more affinity than when fully de-O2

Front 22

2,3-BPG

Back 22

-DECREASES O2 AFFINITY FOR HEMOGLOBIN
-2,3-bisphosphoglycerate
-works at the interface between B1 and B2 chains
-same concentration in RBCs as hemoglobin (~2mM)
-binds preferentially to adult deoxyhemoglobin, stabilizes T form

Front 23

Fetal hemoglobin

Back 23

-2alpha, 2 gamma (gene duplication of the beta chain)
-gamma has 2 fewer positive charges in 2,3-BPG binding site b/c: Ser substitution at His 143
--> reduced affinity for 2,3-BPG, therefore increased affinity for O2
*efficient transfer of O2 from maternal to fetal RBCs*

Front 24

Bohr Effect

Back 24

-hydrogen ions and CO2 increase O2 transporting efficiency
-Proton Effect: favors O2 release
-CO2 effect: favors O2 release

Front 25

Bohr Effect: Proton

Back 25

-acidic pH: amino termini and His side chains are protonated, which locks His beta 146 in a salt bridge with Asp 94
-basic pH: His beta 146 is deprotonated and will not form a salt bridge with Asp 94

Front 26

Bohr Effect: CO2

Back 26

-CO2 reacts with primary amines to form carbamate groups
--> salt bridges within hemoglobin!
-stabilize the T state
-carbamate groups are at the interface of alpha/beta dimers

Front 27

How is CO2 transported in the blood?

Back 27

86% as bicarbonate
14% as carbamate

Front 28

Sickle Cell Anemia

Back 28

-HbS: Val6 instead of glutamate on beta chain
-fibrous aggregates spanning RBC
-clog small capillaries and impair blood flower
-reduced life span --> anemia
-DeoxyHbS has low solubility

Front 29

Sickle cell trait

Back 29

-one copy of HbB, one copy HbS
-resistant to malaria
-malaria caused by plasmodium falciparum

Front 30

alpha-thalassemia

Back 30

-HbH: alpha chain not produced at sufficient quantities
-binds O2 with high affinity and no cooperativity

Front 31

beta-thalassemia

Back 31

-thalassemia major or Cooley anemia
-beta chain is not produced at sufficient quantities
-alpha chains form insoluble aggregates, precipitate inside immature RBC

Front 32

AHSP

Back 32

-alpha-hemoglobin stabilizing protein
-binds to deoxy and oxy alpha-hemoglobin on the same face at beta-hemoglobin
-beta hemoglobin displaces AHSP when it is present