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5| Function of Globular Proteins; and now
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CHAPTER 5: Function of Globular Proteins
Key topics in protein function:
• Reversible binding of ligands is essential
– Specificity of ligands and binding sites
– Ligand binding is often coupled to conformational changes,
sometimes quite dramatic (Induced Fit)
– In multisubunit proteins, conformational changes in one
subunit can affect the others (Cooperativity)
– Interactions can be regulated
• Illustrated examples by:
– Hemoglobin (Hb) antibodies (Ab), and muscle contraction
Interaction with Other Molecules
• Reversible, transient process of chemical equilibrium:
A + B  AB

• A molecule that binds to a protein is called a ligand
– Typically a small molecule
• A region in the protein where the ligand binds is called the
binding site
• Ligand binds via same noncovalent forces that dictate
protein structure (see Chapter 4)
– Allows the interactions to be transient
Binding: Quantitative Description
•
Consider a process in which a ligand (L) binds reversibly to a site in a
protein (P)
P
•
+
ka
L
PL
kd
The kinetics of such a process is described by:
– the association rate constant ka or the dissociation rate constant kd
•
After some time, the process will reach the equilibrium
where the association and dissociation rates are equal
•
The equilibrium composition is
characterized by the equilibrium
constant Ka
ka [P]  [L]  kd [PL]
ka
[PL]
Ka 

[ P ]  [ L] k d
Binding:
Analysis in Terms of the Bound Fraction
• In practice, we can often
determine the fraction of
occupied binding sites (θ)
• Substituting [PL] with Ka[L][P],
we’ll eliminate [PL]

[PL]
[PL]  [P]

K a [L][P]
K a [L][P]  [P]
• Eliminating [P] and rearranging
gives the result in terms of
equilibrium assoc. constant

• In terms of the more commonly
used equilibrium dissoc. constant
[ L]

[ L]  K d
[ L]
[ L] 
1
Ka
Binding: Graphical Analysis
• The fraction of bound sites depends on
the free ligand concentration and Kd
• Experimentally
– Ligand concentration is known
– Kd can be determined graphically or via
– least-squares regression
Graphical representations of ligand
binding. The fraction of ligand-binding
sites occupied, θ, is plotted against the
concentration of free ligand. Both
curves are rectangular hyperbolas. (a)
A hypothetical binding curve for a
ligand L. The [L] at which half of the
available ligand-binding sites are
occupied is equivalent to 1/Ka, or Kd.
The curve has a horizontal asymptote
at θ = 1 and a vertical asymptote (not
shown) at [L] = –1/Ka.
[ L]

[ L]  K d
[L]  [L]total
Example: Oxygen Binding to Myoglobin
When ligand is a gas, binding is
expressed in terms of partial
pressures.
[L]

K d  [L]
pO 2

p50  pO 2
For previous slide:
• Myoglobin. The eight α-helical segments (shown here as
cylinders) are labeled A through H. Nonhelical residues in the
bends that connect them are labeled AB, CD, EF, and so forth,
indicating the segments they interconnect. A few bends,
including BC and DE, are abrupt and do not contain any residues;
these are not normally labeled. (The short segment visible
between D and E is an artifact of the computer representation.)
The heme is bound in a pocket made up largely of the E and F
helices, although amino acid residues from other segments of the
protein also participate.
• Graphical representations of ligand binding. The fraction of
ligand-binding sites occupied, θ, is plotted against the
concentration of free ligand. Both curves are rectangular
hyperbolas. (b) A curve describing the binding of oxygen to
myoglobin. The partial pressure of O2 in the air above the
solution is expressed in kilopascals (kPa). Oxygen binds tightly to
myoglobin, with a P50 of only 0.26 kPa.
Binding: Thermodynamic Connections
• Interaction strength can be expressed as
– association (binding) constant Ka, units M-1
– dissociation constant Kd, units M, Kd = 1/Ka
– interaction (binding) free energy Go, units: kJ/mol
Definitions
– Go = Ho -TSo : enthalpy and entropy
– Ka = [PL]/[P][L]
Kd = [P][L]/[PL]
• Relationships
– Go = -RT ln Ka = RT ln Kd (RT at 25oC is 2.48 kJ/mol)
• Magnitudes
– Strong binding: Kd < 10 nM
– Weak binding: Kd > 10 M
Examples of Binding Strength
Specificity: Lock-and-Key Model
• Proteins typically have high specificity: only certain ligands
bind
• High specificity can be explained by the complementary of
the binding site and the ligand.
• Complementary in
– size,
– shape,
– charge,
– or hydrophobic/hydrophilic character
• “Lock and Key” model by Emil Fisher (1894) assumes that
complementary surfaces are preformed.
+
Specificity: Induced Fit
• Conformational changes may occur upon ligand binding
(Daniel Koshland in 1958)
– This adaptation is called the induced fit
– Induced fit allows for tighter binding of the ligand
– Induced fit allows for high affinity for different
ligands
• Both the ligand and the protein can change their
conformations
+
Globins are oxygen-binding proteins
• Protein side chains lack affinity for O2
• Some transition metals bind O2 well but would generate
free radicals if free in solution
• Organometallic compounds such as heme are more
suitable, but Fe2+ in free heme could be oxidized to
Fe3+
• Solution
– Capture the oxygen molecule with heme that is
protein bound
– Myoglobin is the main oxygen storage protein
– Hemoglobin is a circulating oxygen-binding protein
Structures of Porphyrin and Heme
•
Heme. The heme group is present in
myoglobin, hemoglobin, and many
other proteins, designated heme
proteins. Heme consists of a
complex organic ring structure,
protoporphyrin IX, with a bound
iron atom in its ferrous (Fe2+) state.
(a) Porphyrins, of which
protoporphyrin IX is only one
example, consist of four pyrrole
rings linked by methene bridges,
with substitutions at one or more of
the positions denoted X. (b, c) Two
representations of heme (derived
from PDB ID 1CCR). The iron atom
of heme has six coordination bonds:
four in the plane of, and bonded to,
the flat porphyrin ring system, and
(d) two perpendicular to it.
Structure of Myoglobin. The eight α-helical segments (shown here as
cylinders) are labeled A through H. Nonhelical residues in the bends that
connect them are labeled AB, CD, EF, and so forth, indicating the segments
they interconnect. A few bends, including BC
and DE, are abrupt and do not contain any residues; these are not normally
labeled. (The short segment visible between D and E is an artifact of the
computer representation.) The heme is bound in a pocket made up largely of
the E and F helices, although amino acid residues fromother segments of the
protein also participate.
The heme group viewed
from the side. This view
shows the two coordination
bonds to Fe2+ that are
perpendicular to the
porphyrin ring system. One is
occupied by a His residue,
sometimes called the
proximal His; the other is
the binding site for oxygen.
The remaining four
coordination bonds are in the
plane of, and bonded to, the
flat porphyrin ring system.
Binding of Carbon Monoxide
• CO has similar size and shape to O2; it can fit to the
same binding site
• CO binds over 20,000 times better than O2 because the
carbon in CO has a filled lone electron pair that can be
donated to vacant d-orbitals on the Fe2+
• Protein pocket decreases affinity for CO, but it still
binds about 250 times better than oxygen
• CO is highly toxic as it competes with oxygen. It blocks
the function of myoglobin, hemoglobin, and
mitochondrial cytochromes that are involved in
oxidative phosphorylation
CO vs. O2 Binding to Free Heme
Steric effects caused by ligand binding to the heme of myoglobin. (a)
Oxygen binds to heme with the O2 axis at an angle, a binding conformation
readily accommodated by myoglobin. (b) Carbon monoxide binds to free heme
with the CO axis perpendicular to the plane of the porphyrin ring. When
binding to the heme in myoglobin, CO is forced to adopt a slight angle because
the perpendicular arrangement is sterically blocked by His E7, the distal His.
This effect weakens the binding of CO to myoglobin.
Heme binding to protein affects O2 binding. Steric effects caused by
ligand binding to the heme of myoglobin. (c) Another view of the
heme of myoglobin, showing the arrangement of key amino acid
residues around the heme. The bound O2 is hydrogen-bonded to the
distal His, His E7 (His64), further facilitating the binding of O2.
Could myoglobin transport O2?
pO2 in lungs is about 13 kPa: it sure binds oxygen well
pO2 in tissues is about 4 kPa: it will not release it!
Would lowering the affinity (P50) of myoglobin to oxygen
help? Graphical representations of ligand binding. The fraction of ligand-
binding sites occupied, θ, is plotted against the concentration of free ligand. Both
curves are rectangular hyperbolas. (b) A curve describing the binding of oxygen to
myoglobin. The partial pressure of O2 in the air above the solution is expressed in
kilopascals (kPa). Oxygen binds tightly to myoglobin, with a P50 of only 0.26 kPa.
For effective transport affinity must vary with
•
A sigmoid (cooperative)
binding curve. A sigmoid
binding curve can be viewed
as a hybrid curve reflecting a
transition from a low-affinity
to a high-affinity state.
Because of its cooperative
binding, as manifested by a
sigmoid binding curve,
hemoglobin is more sensitive
to the small differences in
O2 concentration between
the tissues and the lungs,
allowing it to bind oxygen in
the lungs (where pO2 is high)
and release it in the tissues
(where pO2 is low).
pO2
How can affinity to oxygen change?
• Must be a protein with multiple binding sites
• Binding sites must be able to interact with each other
• This phenomenon is called cooperativity
– positive cooperativity
• first binding event increases affinity at
remaining sites
• recognized by sigmoidal binding curves
– negative cooperativity
• first binding event reduces affinity at remaining
sites
The Hill Plot of Cooperativity
• Hill plots for oxygen binding
to myoglobin and
hemoglobin. When nH 5 1,
there is no evident
cooperativity. The maximum
degree of cooperativity
observed for hemoglobin
corresponds approximately to
nH 5 3. Note that while this
indicates a high level of
cooperativity, nH is less than
n, the number of O2-binding
sites in hemoglobin. This is
normal for a protein that
exhibits allosteric binding
behavior.
Two Models of Cooperativity:
Concerted vs.Sequential
Two general models for the interconversion
of inactive and active forms of a protein
during cooperative ligand binding. Although
the models may be applied to any protein—
including any enzyme—that exhibits
cooperative binding, we show here four
subunits because the model was originally
proposed for hemoglobin. (a) In the concerted,
or all-or-none, model (MWC model), all subunits
are postulated to be in the same conformation,
either all  (low affinity or inactive) or all 
(high affinity or active). Depending on the
equilibrium, Keq, between  and  forms, the
binding of one or more ligand molecules (L) will
pull the equilibrium toward the  form.
Subunits
with bound L are shaded. (b) In the sequential
model, each individual subunit can be in either
the  or  form. A very large number of
conformations is thus possible.
Cooperativity is a
special case of allosteric regulation
• Allosteric protein
– Binding of a ligand to one site affects the binding
properties of a different site, on the same protein
– Can be positive or negative
– Homotropic
• Normal ligand of the protein is the allosteric
regulator
– Heterotropic
• Different ligand affects binding of the normal
ligand
• Cooperativity = positive homotropic regulation
Hb binds oxygen cooperatively
• Hb is a tetramer of two subunits (a2b2)
• Each subunit is similar to mb
Sequence Similarity between Hemoglobin and Myoglobin
For previous slide;
The amino acid sequences of whale myoglobin and the α and β
chains of human hemoglobin. Dashed lines mark helix boundaries. To
align the sequences optimally, short gaps must be introduced into both
Hb sequences where a few amino acids are present in the other,
compared sequences. With the exception of the missing D helix in
Hbα, this alignment permits the use of the helix lettering convention
that emphasizes the common positioning of amino acid residues that
are identical in all three structures (shaded). Residues shaded in pink
are conserved in all known globins. Note that the common helix-letterand- number designation for amino acids does not necessarily
correspond to a common position in the linear sequence of amino acids
in the polypeptides. For example, the distal His residue is His E7 in all
three structures, but corresponds to His64, His58, and His63 in the
linear sequences of Mb, Hbα, and Hbβ, respectively. Nonhelical
residues at the amino and carboxyl termini, beyond the first (A) and
last (H) α-helical segments, are labeled NA and HC, respectively.
Where we left off TUESDAY
Subunit Interactions in Hb
Dominant interactions
between hemoglobin
subunits. In this
representation, α subunits
are light and β subunits
are dark. The strongest
subunit interactions
(highlighted) occur
between unlike subunits.
When oxygen binds, the
α1β1 contact changes
little, but there is a large
change at the α1β2
contact, with several ion
pairs broken.
Subunit Interactions: Details; Some ion pairs that stabilize the T state
of deoxyhemoglobin. (a) Close-up view of a portion of a deoxyhemoglobin
molecule in the T state. Interactions between the ion pairs His HC3 and Asp
FG1 of the β subunit (blue) and between Lys C5 of the α subunit (gray) and His
HC3 (its α-carboxyl group) of the β subunit are shown with dashed lines. (Recall
that HC3 is the carboxyl-terminal residue of the β subunit.)
Subunit Interactions: Details; Some ion pairs that stabilize the T
state of deoxyhemoglobin. (b) Interactions between these ion pairs,
and between others not shown in (a), are schematized in this
representation of the extended polypeptide chains of hemoglobin.
R and T States of Hb
• T = Tense state,
– More interactions, more stable
– Lower affinity for O2
• R = Relaxed state,
– Fewer Interactions, more flexible
– Higher affinity for O2
• O2 binding triggers a T  R conformational change
• Conformational change from the T state to the R state
involves breaking ion pairs between the α1-2 interface
R and T States of Hemoglobin The TR transition. In these depictions of
deoxyhemoglobin, as in Figure 5–9, the β subunits are blue and the α subunits
are gray. Positively charged side chains and chain termini involved in ion pairs
are shown in blue, their negatively charged partners in red. The Lys C5 of
each α subunit and Asp FG1 of each β subunit are visible but not labeled. Note
that the molecule is oriented slightly differently than in Figure 5–9. The
transition from the T state to the R state shifts the subunit pairs
substantially, affecting certain ion pairs. Most noticeably, the His HC3
residues at the carboxyl termini of the β subunits, which are involved in ion
pairs in the T state, rotate in the R state toward the center of the molecule,
where they are no longer in ion pairs. Another dramatic result of the TR
transition is a narrowing of the pocket between the β subunits.
Conformational change is triggered by oxygen binding Changes in
conformation near heme on O2 binding to deoxyhemoglobin. The
shift in the position of helix F when heme binds O2 is thought to be
one of the adjustments that triggers the T → R transition.
pH Effect on O2 Binding to Hb
• Actively metabolizing tissues generate H+, lowering the pH of
the blood near the tissues relative to the lungs
• Hb Affinity for oxygen depends on the pH
– H+ binds to Hb and stabilizes the T state
• Protonates His146 which then forms a salt bridge with
Asp94
• Leads to the release of O2 (in the tissues)
• The pH difference between lungs and metabolic
tissues increases efficiency of the O2 transport
• This is known as the Bohr effect
pH Effect on O2 Binding to Hb
• Effect of pH on
oxygen binding to
hemoglobin. The pH
of blood is 7.6 in the
lungs and 7.2 in the
tissues.
Experimental
measurements on
hemoglobin binding
are often performed
at pH 7.4.
Hb and CO2 Export
• CO2 is produced by metabolism in tissues and must be exported
• 15–20% of CO2 is exported in the form of a carbamate on the amino
terminal residues of each of the polypeptide subunits.
• Notice:
– the formation of a carbamate yields a proton which can contribute
to the Bohr Effect
– the carbamate forms additional salt bridges stabilizing the T state
• The rest of the CO2 is exported as dissolved bicarbonate
– Formed by carbonic anhydrase, and also producing a proton
2,3-Bisphosphoglycerate regulates O2 binding
• Negative heterotropic regulator of Hb function
• Present at mM concentrations in erythrocytes
– Produced from an intermediate in glycolysis
• Small negatively charged
molecule, binds to the positively
charged central cavity of Hb
• Stabilizes the T states
2,3-BPG binds to the central cavity of Hb
Binding of BPG to deoxyhemoglobin. (a) BPG binding stabilizes the T
state of deoxyhemoglobin. The negativecharges of BPG interact with
several positively charged groups (shown in blue in this surface contour
image) that surround the pocket between the β subunits on the surface
of deoxyhemoglobin in the T state. (b) The binding pocket for BPG
disappears on oxygenation, following transition to the R state. (Compare
Fig. 5–10.)
2,3-BPG allows for O2 release in the tissues
and adaptation to changes in altitude
•
Effect of BPG on oxygen binding to hemoglobin.
The BPG concentration in normal human blood is
about 5 mM at sea level and about 8 mM at high
altitudes. Note that hemoglobin binds to oxygen
quite tightly when BPG is entirely absent, and the
binding curve seems to be hyperbolic. In reality, the
measured Hill coefficient for O2-binding
cooperativity decreases only slightly (from 3 to
about 2.5) when BPG is removed from hemoglobin,
but the rising part of the sigmoid curve is confined
to a very small region close to the origin. At sea
level, hemoglobin is nearly saturated with O2 in the
lungs, but just over 60% saturated in the tissues, so
the amount of O2 released in the tissues is about
38% of the maximum that can be carried in the
blood. At high altitudes, O2 delivery declines by
about one-fourth, to 30% of maximum. An increase
in BPG concentration, however, decreases the
affinity of hemoglobin for O2, so approximately 37%
of what can be carried is again delivered to the
tissues.
Spectroscopic Detection of
Oxygen Binding to Myoglobin
• The heme group is a strong chromophore that absorbs both in
ultraviolet and visible range
• Ferrous form (Fe2+ ) without oxygen has an intense Soret band at
429 nm
• Oxygen binding alters the electronic properties of the heme, and
shifts the position of the Soret band to 414 nm
• Binding of oxygen can be monitored by UV-Vis spectrophotometry
• Deoxyhemoglobin (in venous blood) appears purplish in color and
oxyhemoglobin (in arterial blood) is red
Sickle-cell anemia is due to
a mutation in hemoglobin
• Glu6  Val in the  chain of Hb
• The new Valine side chain can bind to a
different Hb molecule to form a strand
• This sickles the red blood cells
• Untreated homozygous individuals
generally die in childhood
• Heterozygous individuals exhibit a
resistance to malaria
Formation of Hb Strands in Sickle-Cell Anemia
Normal and sickle-cell
hemoglobin. (a) Subtle
differences between the
conformations of hemoglobin
A and hemoglobin S result
from a single amino acid
change in the β chains. (b) As
a result of this change,
deoxyhemoglobin S has a
hydrophobic patch on its
surface, which causes the
molecules to aggregate into
strands that align into
insoluble fibers.
Two Types of Immune Systems
• Cellular immune system
- targets own cells that have been infected
- also clears up virus particles and infecting bacteria
- key players: Macrophages, killer T cells (Tc),
and inflammatory T cells (TH1)
• Humoral “fluid” immune system
- targets extracellular pathogens
- can also recognize foreign proteins
- makes soluble antibodies
- keeps “memory” of past infections
- key players: B-lymphocytes and helper T-cells (TH2)
Cellular Immune System
• Antibodies bind to fragments displayed on the surface of invading
cells
• Phagocytes: specialized cells that eat invaders
• Macrophages: large phagocytes that ingest bacteria that are
tagged by antibodies
Humoral Immune System
• Vertebrates also fight infections with soluble antibodies
that specifically bind antigens
– Antigens are substances that stimulate production of
antibodies
•
•
•
•
Typically macromolecular in nature
Recognized as foreign by the immune system
Coat proteins of bacteria and viruses
Surface carbohydrates of cells or viruses
– Antibodies are proteins that are produced by B cells and
specifically bind to antigens
• Binding will mark the antigen for destruction or interfere with its
function
• A given antibody will bind to a small region (epitope) of the antigen
• One antigen can have several epitopes
Antibodies: Immunoglobulin G
• Composed of two heavy chains and two light chains
• Composed of constant domains and variable domains
• Light chains: one constant and one variable domain
• Heavy chains: three constant and one variable domain
• Variable domains of each chain make up antigenbinding site (two/antibody)
• Variable domains contain regions that are
hypervariable (specifically the antigen-binding site)
• Confers high antigen specificity
Antibodies: Immunoglobulin G
• Immunoglobulin G. (a) Pairs
of heavy and light chains
combine to form a Y-shaped
molecule. Two antigenbinding sites are formed by
the combination of variable
domains from one light (VL)
and one heavy (VH) chain.
Cleavage with papain
separates the Fab and Fc
portions of the protein in
the hinge region. The Fc
portion of the molecule also
contains bound carbohydrate
(shown in (b)).
Antibodies: Immunoglobulin G
A ribbon model of the first
complete IgG molecule to be
crystallized and structurally
analyzed. Although the
molecule has two identical
heavy chains (two shades of
blue) and two identical light
chains (two shades of red), it
crystallized in the asymmetric
conformation shown here.
Conformational flexibility may
be important to the function of
immunoglobulins.
Antigens bind via induced fit; Antigen binding causes significant
structural changes to the antibody
Induced fit in the binding of an antigen to IgG. The molecule here, shown
in surface contour, is the Fab fragment of an IgG. The antigen is a small
peptide derived from HIV. Two residues in the heavy chain (blue) and one
in the light chain (red) are colored to provide visual points of reference.
(a) View of the Fab fragment in the absence of antigen, looking down on
the antigen-binding site. (b) The same view, but with the Fab fragment in
the “bound” conformation; the antigen is omitted to provide an
unobstructed view of the altered binding site. Note how the binding
cavity has enlarged and several groups have shifted position. (c) The
same view as (b), but with the antigen in the binding site, pictured as a
red stick structure.
The BCR
The membrane-bound form of an antibody may be called a surface
immunoglobulin (sIg) or a membrane immunoglobulin (mIg). It is part of
the B cell receptor (BCR), which allows a B cell to detect when a
specific antigen is present in the body and triggers B cell activation.
The BCR is composed of surface-bound IgD or IgM antibodies and
associated Ig-α and Ig-β heterodimers, which are capable of signal
transduction. A typical human B cell will have 50,000 to 100,000
antibodies bound to its surface. Upon antigen binding, they cluster in
large patches, which can exceed 1 micrometer in diameter, on lipid
rafts that isolate the BCRs from most other cell signaling receptors.
These patches may improve the efficiency of the cellular immune
response. In humans, the cell surface is bare around the B cell
receptors for several hundred nanometers, which further isolates the
BCRs from competing influences.
Antibody specificity is an
important analytical reagent
Protein Interactions
Modulated by Chemical Energy
• Use of chemical energy (ATP) can cause conformational
changes in proteins, generally required for their function
• Especially in motor proteins
– Control movement of cells and organelles within cells
• Allows for spatial and temporal regulation of interactions
Muscle Structure
• Muscle fiber: large, single, elongated, multinuclear cell
• Each fiber contains about 1,000 myofibrils
Skeletal muscle. (a) Muscle fibers consist of single, elongated, multinucleated
cells that arise from the fusion of many precursor cells. The fibers are made
up of many myofibrils (only six are shown here for simplicity) surrounded by
the membranous sarcoplasmic reticulum. The organization of thick and thin
filaments in a myofibril gives it a striated appearance. When muscle contracts,
the I bands narrow and the Z disks come closer together, as seen in electron
micrographs of (b) relaxed and (c) contracted muscle.
Myofibrils contain
thick filaments of myosin
Myofibrils contain
thin filaments of actin
Myosin thick filaments slide along
actin thin filaments
Myosin thick filaments slide along
actin thin filaments
From previous slide
5-29b,c Skeletal muscle. (a) Muscle fibers consist of single,
elongated, multinucleated cells that arise from the fusion of
many precursor cells. The fibers are made up of many myofibrils
(only six are shown here for simplicity) surrounded by the
membranous sarcoplasmic reticulum. The organization of thick
and thin filaments in a myofibril gives it a striated appearance.
When muscle contracts, the I bands narrow and the Z disks
come closer together, as seen in electron micrographs of (b)
relaxed and (c) contracted muscle.
FIGURE 5–30a Muscle contraction. Thick filaments are bipolar
structures created by the association of many myosin molecules.
(a) Muscle contraction occurs by the sliding of the thick and
thin filaments past each other so that the Z disks in
neighboring I bands draw closer together.
Actomyosin Cycle
• Muscle contraction occurs through a series of
conformational changes to protein structure due to
binding, hydrolysis, and release of ATP and ADP
• Cycle has four steps
1. ATP binds to myosin myosin dissociates from actin
2. ATP is hydrolyzed  a conformational change of myosin
3. Myosin re-connects to the actin filament at a different location
 release of Pi
4. Release of Pi Power stroke where myosin returns to initial state;
shifting actin filament relative to the myosin tail  release of ADP
Actomyosin Cycle
•
Molecular mechanism of muscle
contraction. Conformational
changes in the myosin head that
are coupled to stages in the ATP
hydrolytic cycle cause myosin to
successively dissociate from one
actin subunit, then associate
with another farther along the
actin filament. In this way the
myosin heads slide along the thin
filaments, drawing the thick
filament array into the thin
filament array (see Figure 530).
Regulation of muscle contraction
•
•
Availability of myosin-binding sites on actin is regulated by troponin and
tropomyosin
– avoids continuous muscle contraction
Nerve impulse triggers release of Ca2+
– Causes conformational changes to tropomyosin-troponin complex
exposing myosin-binding sites
From pervious slide
Regulation of muscle contraction by tropomyosin and troponin. Tropomyosin
and troponin are bound to F-actin in the thin filaments. In the relaxed
muscle, these two proteins are arranged around the actin filaments so as to
block the binding sites for myosin. Tropomyosin is a two-stranded coiled coil
of α helices, the same structural motif as in α-keratin (see Fig. 4–11 ). It
forms head-to-tail polymers twisting around the two actin chains. Troponin
is attached to the actin-tropomyosin complex at regular intervals of 38.5
nm. Troponin consists of three different subunits: I, C, and T. Troponin I
prevents binding of the myosin head to actin; troponin C has a binding site
for Ca2+; and troponin T links the entire troponin complex to tropomyosin.
When the muscle receives a neural signal to initiate contraction, Ca2+ is
released from the sarcoplasmic reticulum (see Fig. 5–29a) and binds to
troponin C. This causes a conformational change in troponin C, which alters
the positions of troponin I and tropomyosin so as to relieve the inhibition by
troponin I and allow muscle contraction.
In the words of Porky Pig, That’s all folks!