Download Chapter 5 Protein Function

Chapter 5
Protein Function
Taq DNA Polymerase
Protein Function - Definitions
•
A Ligand is a molecule that binds specifically and reversibly to a larger one.
– Examples of ligand interactions include:
• Hormones/Receptors
• Antigen/Antibody
• Substrate/Enzyme
•
These molecules interact with proteins at the binding site, which is
complimentary to the ligand in size, charge, hydrophobicity and shape
•
Proteins are highly flexible, allowing them to change their conformations if
needed
– The interaction is highly specific.
– The flexibility allows the fit between the binding partners to be an induced fit
(conformational change) or a lock and key fit (no change)
•
Interactions between ligands and proteins is usually regulated through a different
binding interaction
•
For enzymes, the ligand is termed the substrate and the binding site is called the
catalytic site or active site
Chapter 5
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1
Protein Function
Ligand Binding is Similar to Ionization
• The term Kd is the
dissociation constant and is
the inverse of the Keq (or the
association constant, Ka)
• As the Kd value decreases in
value, the affinity of the ligand
for the protein is increasing.
• For most binding equilibria in
cells, the [L] is much greatly
than the number of binding
sites
K eq =
• Therefore, the binding of L to
the protein does not
appreciably effect the [L] so it
is considered a constant
[M - L]
[M][L]
Kd =
Kd =
[M][L]
[M - L]
1
K eq
Chapter 5
3
Protein Function
Fraction of Binding Sites Occupied (θ)
θ=
[L]
Bound
[M]tot
=
[M - L]
[M] + [M - L]
[M - L] = [M][L]
Kd
θ=
[L]
K + [L]
The Langmuir Isotherm
d
Chapter 5
4
2
Protein Function
Kd is a Measure of Binding Strength
Chapter 5
5
Reversible Binding of a Protein to a Ligand
Classic Examples:
O2 Binding to Myoglobin and Hemoglobin
•
•
•
•
Myoglobin
Reversible binding and
storage of O2 in
muscles
Monomeric protein
structure (16.7 kDa)
153 AAs
Contains one heme
group
Chapter 5
•
•
•
•
Hemoglobin
Reversible binding and
transport of O2 in
erythrocytes
Tetrameric protein
structure (α2β2) (64.5 kDa)
α chain 141 AAs; β chain
145 AAs
Contains four heme
groups
6
3
Reversible Binding of a Protein to a Ligand
Heme: Protoporphyrin IX and Fe2+
Chapter 5
7
Reversible Binding of a Protein to a Ligand
The Hexa-Coordination of Fe2+ in Mb
#1-4 are N-atoms
from the
porphyrin ring
#5-6 are to…?
Chapter 5
8
4
Reversible Binding of a Protein to a Ligand
Histidines Coordinate the Iron and Help Bind O2
Binding orientation with free heme
Distal
His 64
How does the Histidine help
with the binding of the O2
molecule?
His 93
Proximal
How would this Histidine
affect CO binding?
Chapter 5
9
Reversible Binding of a Protein to a Ligand
O2 Binding to Myoglobin
θ=
[O ]
K + [O ]
2
d
θ=
2
[O ]
[O ] + [O ]
2
2
2
0.5
θ=
Chapter 5
pO 2
pO 2 + P50
10
5
Reversible Binding of a Protein to a Ligand
Now Let’s Look at Hemoglobin
• Unlike myoglobin, which is a monomer, hemoglobin is a tetramer.
• It has two identical alpha and two identical beta subunits; its quaternary
structure is thus an α2β2 heterotetramer
• The primary structures of all four subunits are similar to each other, and
to myoglobin, suggesting evolution by gene duplication
Chapter 5
11
Reversible Binding of a Protein to a Ligand
Oxygen Binding to Hemoglobin
• Sigmoidal binding
curve suggesting
allosteric effects for
Hb [Gr. “allo”=other,
“steros”=shape]
• Meaning that
something is effecting
the binding of O2 to
the four subunits of
Hb
Chapter 5
12
6
Reversible Binding of a Protein to a Ligand
O2 Binding in Hb Leads to Local Conformational
Changes Near the Heme…
No oxygen
Chapter 5
What has happened to the heme plane upon O2 binding?
13
Reversible Binding of a Protein to a Ligand
….and Global Conformational Changes in the Tetramer
1BBB
Fig. 5-10
Stabilized without oxygen Stabilized by oxygen binding
by numerous ion pairs
1HGA
Chapter 5
Note shift in HC3 Histidines!
14
7
Reversible Binding of a Protein to a Ligand
A Closer View of Some Stabilizing
Ionic Contacts in the T-State (Deoxyferrous)
Chapter 5
15
Reversible Binding of a Protein to a Ligand
Many Ionic Contacts Help Stabilize Hb’s T-State
Chapter 5
16
8
Reversible Binding of a Protein to a Ligand
•
•
•
•
Cooperativity Leads to a Sigmoid
(Allosteric) Binding Curve
Deoxyhemoglobin: T-state
R-state
picks up O2 in the lungs,
where pO2 is high,
Converting Hb into the Rstate, so it readily picks up
more O2
As loaded oxyhemoglobin (RT-state
state) moves back to tissues,
where pO2 is lower, it begins
to lose its O2,
Which facilitates the transition
back to the T-state
Chapter 5
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Reversible Binding of a Protein to a Ligand
Cooperativity (Allosteric) Binding
[M][L]
=
[M - L ]
n
Kd
[L]
K + [L ]
n
n
θ=
n
The Langmuir Isotherm
d
• Here, θ is defined as the number of moles of ligand bound
per mole of macromolecule and can vary from 0 to n.
Chapter 5
18
9
Reversible Binding of a Protein to a Ligand
The Hill Plot
• If there are multiple binding sites in a protein, we need to determine
if the binding of ligands is cooperative.
• For many proteins (like Hb), the binding of the first ligand can
change the affinity for the second ligand. For our two site example:
Positive Cooperativity if K2 > K1 i.e. the second binding is higher in affinity
Negative Cooperativity if K2 < K1 i.e. the fist binding is higher in affinity
• We can use a Hill Plot to determine the cooperativity and the
average K-d value for the binding. This Hill equation is derived from
ν and θ and the [L]:
Chapter 5
⎛ θ ⎞
log⎜
⎟ = log K n + nh log [L] The Hill Equation
⎝ 1- θ ⎠
19
Reversible Binding of a Protein to a Ligand
The Hill Plot
⎛ θ ⎞
log⎜
⎟ = log K n + nh log [L] The Hill Equation
⎝ 1- θ ⎠
• Kn is the average Kd value for all the binding sites and nh
is the Hill coefficient which characterizes the degree
of cooperativity:
nh > 1
The system shows positive cooperativity
nh = N
The cooperativity is infinite with N total number of
ligand bound
nh = 1
The system is non-cooperative
nh < 1
The systems shows negative cooperativity
Chapter 5
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10
Reversible Binding of a Protein to a Ligand
The Hill Plot
⎛ θ ⎞
log⎜
⎟ = log K n + nh log [L] The Hill Equation
⎝ 1- θ ⎠
By plotting the log (θ/1-θ) versus the log [L]
can yield a linear or curved plot. The shape
of the plot is dependent on:
– When cooperativity is not complete
(i.e., n-h < N), the Hill plot is not linear.
– At the extremes of [L], the line has a
slope of ~1.0.
– At low ligand concentrations, there is
no cooperativity. Thus the Hill plot will
represent single-site binding (binding
of the first ligand molecule).
– At high ligand concentrations, all sites
are filled but one. Thus this region of
the Hill plot should also represent
single-site binding for the last ligand.
Chapter 5
21
Reversible Binding of a Protein to a Ligand
Sigmoid Binding Curve
Notice how the Kd
changes from the highaffinity T-state to the
low-affinity R-state.
Why would pure “R”
or “T” forms function
less well in
erythrocytes??
Kd
Chapter 5
Kd
22
11
Reversible Binding of a Protein to a Ligand
Hemoglobin and the Bohr Effect
• In its R-state, Hb is capable of 66% saturation by O2
• Not too good if you want to function at full capacity
• The Bohr Effect deal with the effect of pH and CO2
concentration on the binding and release of oxygen by
hemoglobin
• The interaction of H+ and CO2 with Hb results in allosteric
changes in the tetramer
• These changes interrelate the physiological parameters in
tissues, blood, and lungs (pH!) with the ability of Hb to bind
oxygen
• This effect results in the movement of H+ and CO2 away
from tissue and back to the lungs and the movement of O2
in the opposite direction.
Chapter 5
23
Reversible Binding of a Protein to a Ligand
Carbon Dioxide, a product of Respiration, is an Acid!
• Carbon dioxide released in
respiring tissues is insoluble
until hydrated and converted to
bicarbonate (VERY slow rxn!)
• In erythrocytes, the rate of this
reaction is increased by the
enzyme carbonic anhydrase
• But this releases equivalent
amounts of H+ (causing the pH
in the tissues and blood to fall)
• This process is reversed in the
lungs, but requires that protons
be available in an area of higher
pH
So, where to they come from?
Chapter 5
24
12
Reversible Binding of a Protein to a Ligand
Blo
od
pla
sm
a
• The binding of H+ and CO2
are inversely related to the
binding of O2
Lu
ng
s
pH Dependence of O2 Binding to Hemoglobin:
The “Bohr Effect”
• Do the Kd values reflect
this?
ss
u
Ti
• At high pH (lungs): Low
[H+] & [CO2] so high affinity
for O2
es
• At low pH (peripheral
tissues): High [H+] & [CO2]
so low affinity for O2
Chapter 5
25
Reversible Binding of a Protein to a Ligand
Recall Those Stabilizing Ionic Contacts in the Deoxy T-State
Fig. 7-9
• H+ and O2 do not bind in the same
site.
• When protonated, His HC3 (H146)
forms an ion pair with Asp FG1
(D94) that helps to stabilize the Tstate of Hb
• This interaction gives H146 a higher pKa than normal and thus it
is readily protonated at pH values found in tissues & the blood
• Back at the lungs, the pH rises and H146 is deprotonated,
meaning the ion pair cannot form and H146 moves into its Rstate location.
What does the stabilization of the T state mean for O2 binding?
Chapter 5
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13
Reversible Binding of a Protein to a Ligand
Hemoglobin interaction with CO2
• The amino-termini of the four chains become carbamylated
at the low pH (7.2) in the tissues:
O
C
O
H+
H
+ H2N
C
C
R O
Amino-terminal
residue
-
H
O
C
H
N
C
C
O
R O
Carbamino-terminal
residue
• The binding of CO2 to the amino group results in the release
of H+, which adds to the Bohr Effect
• These anionic groups can also form additional salt bridges
that help stabilize the T-state of Hb
• This interaction helps to transport ~ 20% of the CO2 formed
in the tissues to the lungs and kidneys
Chapter 5
Where does the rest go?
27
Reversible Binding of a Protein to a Ligand
Oxygen Carrying Capacity of Hemoglobin
• Carrying capacity (oxygen delivery)
between 13 kPA (100 Torr) O2 and 4 kPA
(20 Torr):
– Hemoglobin (non-cooperative) …….. 38%*
– Hemoglobin (cooperative) ………….. 66%
– Hemoglobin + Bohr pH effect ………. 77%
– Hemoglobin + Bohr pH effect + CO2.. 88%
Chapter 5
*Percent of maximum carrying capacity.
(See Stryer: page 273)
28
14
Reversible Binding of a Protein to a Ligand
O2 Binding is Also Regulated by BPG
D-2,3-bisphosphoglycerate (BPG) can bind
in a small pocket in the
center of the hemo-globin
tetramer – but only to the
T-state!
• This hole is lined with
cationic AAs that interact
with the anionic BPG
• Only one BPG per Hb
tetramer
• The conformation change
in the R-state closes up
the internal “hole”, thereby
excluding BPG from
binding
O-
•
-
O
P
C
CH
O
O
O
OH2
C O
O
P
O-
O-
T
Fig. 5-18
R
Chapter 5
29
Reversible Binding of a Protein to a Ligand
The circular interplay of factors associated with Hb
in binding or releasing oxygen:
•
•
•
•
•
Why is the pH in the lungs high?
What does that do to protonation of His HC3?
How does that help get rid of CO2?
How does eliminating CO2 help O2 binding?
How would the release of O2 vary in tissues undergoing
strenuous aerobic exercise?
• How does BPG help in acclimatization to high altitude?
• What is the main difference between adult and fetal
Hb?
See text discussion, pgs. 170-172
Chapter 5
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15
Sickle-Cell Anemia
SCA is due to a mutation
that converts Glu-6 in the βchains to Val (E6V mutation).
• This substitution creates hydrophobic “sticky” patches on
the normally charged surface of the β-chains.
• The oxygenated molecules are soluble, but upon deoxygenation, the conformation of HbS differs considerably
from HbA, and it aggregates into insoluble fibers (as
diagrammed on the next slide).
• These fibers deform the RBCs into spiny or sickle-shaped
cells.
Chapter 5
31
The Tortuous Tale of Hemoglobin S
Chapter 5
32
16
Reversible Binding of a Protein to a Ligand
Some questions about sickle cell anemia:
• Where in the tissues will “sickling” occur?
• Why is it homozygous lethal?
• Does it tell you anything about conformation changes in hemoglobin?
• Why should heterozygous individuals be
more resistant to malaria?
• What does sickle cell anemia tell you about
the importance of protein primary structure?
Chapter 5
33
Reversible Binding of a Protein to a Ligand
Key Points About Ligand Binding
• Proteins may change conformation when ligands bind
(allosteary); in a multimer, other subunits can be affected
• For a monomer like myoglobin, the simple reversible binding of
oxygen may be described by a dissociation constant, KD
• Hemoglobin is an α2β2 heterotetramer, in which oxygen binding
shifts the stability of the T-state to the R-state
• Oxygen binding to hemoglobin causes both allosteric
conformational changes and cooperative effects due to subunitsubunit interactions between the subunits
• Hemoglobin also binds H+ and CO2, forming ion pairs that lessen
the affinity for oxygen (Bohr effect); the binding of 2,3bisphosphoglycerate also prevents efficient oxygen binding
• A genetic mutation of β E6V causes sickle-cell anemia because of
the hydrophobic patch that leads to aggregation of the
deoxyhemoglobin form.
Chapter 5
34
17
The Immune System and Immunoglobulins
• Oxygen binding proteins demonstrated how
conformations of Mb and Hb affect and are affected by
the binding of small ligands to the heme group
• However, most protein-ligand interactions do not involve
a prosthetic group
• Rather, the binding site of a ligand within a protein is like
that seen for BPG in Hb, a cleft lined with complimentary
AAs
• The immune system utilizes this type of complimentary
binding to distinguish “self” from “non-self” then to
destroy the “non-self” entities.
Chapter 5
35
The Immune System and Immunoglobulins
• Immunity is controlled by a variety of leukocytes (white
blood cells) all developed from undifferentiated stem cells in
the bone marrow.
• These cells can leave the
bloodstream and patrol the
tissues, looking for molecules
that might indicate an infection
• There are two complimentary
immune systems:
– Humoral IS is directed at bacterial
and extracellular infections
– Cellular IS destroys host cells
infected by viruses, and some
parasites and foreign tissues
Chapter 5
36
18
The Immune System and Immunoglobulins
• The proteins at the center of the Humoral IS are the
immunoglobulins (Ig) or the antibodies
• These proteins bind bacteria, viruses or large molecules identified
as “non-self” and target them for destruction
• These proteins are produces by the B Lymphocytes (B cells) and
make up 20% of the blood protein
• The proteins at the center of the Cellular IS are the cytotoxic T
cells (TC; killer T cells) and the helper T cells (TH cells)
• Recognition of infected cells or parasites involves proteins on the
surface of the killer T cells called T-cell receptors
• The TH cells produce soluble signaling proteins called cytokines
that interact with other proteins and cells of the immune system
Chapter 5
37
The Immune System and Immunoglobulins
• Any substance capable of initiating an immune response is
called an antigen (antibody generator)
• This antigen could be a virus, a bacterial cell wall component,
an individual protein or any other foreign entities
• An antibody or T-cell receptor binds only to a particular
portion of the antigen called its epitope
• Some epitopes include:
– Proteins containing an N-terminus formylated methionine (only found
in procaroytes)
– Cell exterior molecules and/or features such as flagella,
lipopolysaccharides, and cell wall components
– Short sequences of bacterial DNA: the unmethylated “CpG motif”
Chapter 5
38
19
The Immune System and Immunoglobulins
Immunological Memory
• The adaptive immune system, like the nervous
system, can remember prior experiences.
• This is why we develop lifelong immunity to
many common infectious diseases after our
initial exposure to the pathogen, and it is why
vaccination works.
Fig 24-10, Molecular Biology of the Cell,
4th Ed., Alberts et al.
• When an animal is immunized with antigen A,
an immune response appears after several
days, rises rapidly and exponentially, and then,
more gradually, declines.
• This is the characteristic course of a primary immune response, occurring on
an animal's first exposure to an antigen.
• If the animal is reinjected with antigen A, it will usually produce a secondary
immune response that is very different from the primary response: the lag
period is shorter, and the response is greater.
• These differences indicate that the animal has "remembered" its first exposure
to antigen A.
Chapter 5
39
The Immune System and Immunoglobulins
Immunological Memory
• In an adult animal, the peripheral lymphoid organs contain
a mixture of cells in at least three stages of maturation:
naïve cells, effector cells and memory cells.
• When naïve cells encounter antigen for the first time,
some of them are stimulated to proliferate and
differentiate into effector cells, which are actively
engaged in making a response
• Effector B cells secrete antibody, while effector T cells kill
infected cells or help other cells fight the infection.
• Instead of becoming effector cells, some naïve cells are
stimulated to multiply and differentiate into memory cells
cells that are not themselves engaged in a response but
are more easily and more quickly induced to become
effector cells by a later encounter with the same antigen.
• Memory cells, like naïve cells, give rise to either effector
cells or more memory cells
Chapter 5
40
Fig 24-11, Molecular Biology of the Cell, 4th Ed., Alberts et al.
20
The Immune System and Immunoglobulins
Major Histocompatibility Complex
• The immune systems must not only be able to identify and destroy
“non-self” but also recognize and NOT destroy “self”
• Detection of protein antigens in the host is mediated by Major
Histocompatibility Complex (MHC)
– MHC bind peptide fragments of digested proteins and present them on
the outside surface of the cell
– These peptides are normally “self” but during a viral infection, viral
proteins are also digested and presented
– These “non-self” peptides serve as antigens for T-cell receptors
resulting in the launch of the immune response.
• There are two types of MHCs: Class I and Class II
– They differ in the type of cell on which they present antigens
Chapter 5
41
The Immune System and Immunoglobulins
Major Histocompatibility Complex
• Class I MHCs are found on the surface of
nearly all vertebrate cells
• These are highly polymorphic proteins with
each individual producing up to 6 class I
MHCs
• They display peptides derived from the
breakdown of protein that occur randomly
within the cell
• These complexes serve as targets for the
T-cell receptors of TC cells
• These proteins are heterodimers with a
small, invariant β chain and a highly
polymorphic α chain
• Antigen binding occurs in the hypervariant
region on the outside of the cell
Chapter 5
42
21
The Immune System and Immunoglobulins
Major Histocompatibility Complex
• Class II MHCs occur on the surfaces of a
few types of specialized cells, including
macrophages and B lymphocytes that
take up foreign antigens
• These are also highly polymorphic
proteins with each individual producing up
to 12 class II MHCs
• They display peptides derived from the
breakdown of external proteins ingested
by the cells
• These complexes serve as targets for the
T-cell receptors of TH cells
• These proteins are heterodimers with
highly variant β and α chains
• Antigen binding occurs in the amino
terminal region on the outside of the cell
Chapter 5
43
The Immune System and Immunoglobulins
Immunoglobulin G (IgG)
• Immunoglobulin G is the major class of
antibody, and one of the most abundant
proteins in the blood serum
• IgG has heterotetramer, with two heavy
chains and two light chains
• These chains are linked to one another by
disulfide bonds into a complex with a MW
of 150 kDa
• The constant domains all have a
characteristic structure called the
immunoglobulin fold
• The variable domain (one heavy, one
light) creat the antigen-binding site.
Chapter 5
44
22
The Immune System and Immunoglobulins
Other Immunoglobulins
• There are five classes of Ig, each
having a characteristic type of
heavy chain
• IgA is found primarily in
secretions (saliva, tears, etc)
IgA
• IgM is the first Ig prepared by the
B lymphocytes and is the major Ig
in the initial stages of the immune
response
~220 AA
~440 AA
Chapter 5
45
The Immune System and Immunoglobulins
IgG and the Secondary Immune Response
•
IgG is the major antibody of the secondary
immune response, which is initiated by the
memory B lymphocytes
•
When IgG binds to an invading bacterium or
virus, it activates certain leukocytes such as
macrophages to ingest and destroy the invader
•
A different set of cell surface receptors on the
surface of the macrophages recognizes the Fc
region of the IgG
•
These receptors bind to the pathogen bound
IgGs resulting in phagocytosis
Chapter 5
46
23
The Immune System and Immunoglobulins
Applications for Antibodies
• Affinity Chromatography
• Analytical Assays
– These assays are used to detect, quantify and
characterize antigens
•
•
•
•
•
Immunofluorescence (IFA)
Immunoblotting (Western Blotting)
Immunoprecipitation
Radioimmunosorbant assay (RIA)
Enzyme-linked immunosorbent assay (ELISA)
– All these assays have two main components:
• Formation of the antigen-antibody complex
• Detection of this complex
Chapter 5
47
The Immune System and Immunoglobulins
Immunoprecipitation
•
Generally a qualitative method used to
detect a particular antigen in a biological
sample
•
Can be used in conjunction with SDS-PAGE
to determine the molecular weight of the
target antigen
•
Steps include:
– Labeling of the antigen (optional)
– Solubilization of the antigen
– Incubation followed by complex formation with
the antibody
– Isolation of antigen-antibody complex
– Electrophoresis, fluorescence or other
detection method
Chapter 5
Protein A (or G) are proteins
isolated from Staphylococcus
areus or Streptococcus that bind
to the Fc region of IgG
48
24
The Immune System and Immunoglobulins
Immunoblotting (The Western Blot)
• Generally a qualitative method used to detect a particular protein in
a biological sample
• Used in conjunction with electrophoresis (usually SDS-PAGE) for
protein separation
• Steps include:
– Antigen electrophoresis
– Transfer to membrane
– Blocking of membrane
– Incubation with primary antibody
followed by wash
– Incubation with enzyme-linked
secondary antibody
– Develop with substrate
Chapter 5
49
The Immune System and Immunoglobulins
Immunofluorescence
• This technique is used to localize antigens to a specific
cell type of subcellular compartment
• Steps include:
– Preparation of cells or tissue
– Incubation with primary antibody
– Wash to remove non-bound 1° antibody
– Incubate with secondary antibody
(with fluorophore attached)
– Wash again to excess 2° antibody
– Detect with UV light
Fluorophores include: Fluorescein and rhodamine
Chapter 5
50
25
The Immune System and Immunoglobulins
ELISA (Enzyme-Linked Immunosorbant Assay)
•
Can be a qualitative or quantitative
method used to detect a particular
protein in a biological sample
•
Steps include:
– Antigen binding to plate
• Binding is due to a hydrophobic
interaction between the antigen and
the plastic.
– Blocking to eliminate non-specific
binding
– Incubation with primary antibody
followed by wash
– Incubation with enzyme-linked
secondary antibody
– Develop with substrate
Chapter 5
51
Actin, Myosin and Molecular Motors
• Protein-based molecular motors are responsible for the
movement of organisms and cells
• These motors are usually fueled by chemical energy
derived from the hydrolysis of Adenosine triphosphate
(ATP)
• Large groups of these motor proteins undergo cyclic
conformational changes that accumulate into a unified,
directional force
• Interactions among motor proteins feature
complimentary arrangements of ionic, hydrogen
bonding, hydrophobic and van der Waals interactions
Chapter 5
52
26
Actin, Myosin and Molecular Motors
• The major motor proteins found in your muscles are actin
and myosin
– Actin
• 5% of total protein
• 20% of vertebrate muscle mass
• G-Actin has 375 amino acids = 42
kDa
• G-Actins associate to form
polymers (F-actin)
• These polymers associate with additional proteins to form filaments
• Each actin monomer binds and then hydrolyzes an ATP molecule to
ADP
• This hydrolysis functions only in the formation of the actin filaments not
in the muscle contraction
Chapter 5
53
Actin, Myosin and Molecular Motors
• The major motor proteins found in your muscles are actin
and myosin
– Myosin
• Hexamer of two heavy subunits
(220 kDa) and four light subunits
(20 kDa)
• At the amino terminus, a large
globular domain is responsible for
ATP hydrolysis for muscle
contraction
• Myosin molecules associate to
form thick filaments, which serve
as the core of the contractile unit
Chapter 5
54
27
Actin, Myosin and Molecular Motors
•
•
•
The interaction between myosin and actin involves weak
bonds.
When ATP is not bound to myosin, a face on the myosin
head is bound tightly to actin
Binding of ATP to myosin results in release from actin and
the beginning of the contractile cycle
Chapter 5
55
28