12/02/2008biochemistry: muscles muscle contraction andy howard introductory biochemistry 2 december...

12/02/2008Biochemistry: Muscles

Muscle Contraction

Andy HowardIntroductory Biochemistry

2 December 2008

12/02/2008Biochemistry: Muscles of 46

Chemistry of muscle contraction The most impressive movement phenomenon in mesoscopic organisms is muscle movement. It does have a biochemical basis, which we’ll explore today


What we’ll discuss Skeletal muscle physiology

Thin filaments: actin, tropomyosin, troponin

Thick filaments: myosin

Sliding filament model

Dystrophin and cytoskeletal structure

Coupling of ATP hydrolysis to conformational changes in myosin

Myosin & kinesin Calcium channels and troponin C

Smooth muscle


Essential Question

How can biological macromolecules, carrying out conformational changes on the microscopic, molecular level, achieve these feats of movement that span the molecular and macroscopic worlds?

We’ll look at the specifics of muscle contraction, which is an excellent example of this phenomenon

Note that Tom Irving, on our faculty, is a world-recognized expert on muscle physiology

QuickTime™ and a decompressor

are needed to see this picture.

Prof. Thomas C. Irving


Skeletal Muscle Cell

T-tubules enable the sarcolemmal membrane to contact the ends of the myofibril


What are t-tubules and SR for?

The morphology is all geared to Ca2+ release and uptake!

Nerve impulses reaching the muscle produce an "action potential" that spreads over the sarcolemmal membrane and into the fiber along the t-tubule network


t-tubules and SR, continued The signal is passed across the triad junction and induces release of Ca2+ ions from the SR

Ca2+ ions bind to sites on the fibers and induce contraction; relaxation involves pumping the Ca2+ back into the SR


Molecular mechanism of contractionBe able to explain the EM in Figure 16.12

in terms of thin and thick filaments Thin filaments are composed of actin polymers

F-actin helix is composed of G-actin monomers

F-actin helix has a pitch of 72 nm But repeat distance is 36 nm Actin filaments are decorated with tropomyosin heterodimers and troponin complexes

Troponin complex consists of: troponin T (TnT), troponin I (TnI), and troponin C (TnC)


Myo- fibrils

Hexagonal arrays shown(fig. 16.12)


Actin monomer

One domain on each side(16.13)


Actin helices Pitch = 72nm

Repeat = 36 nm

Fig.16.14


Thin filament

Tropomyosin coiled coil winds around the actin helix

Each TM dimer interacts with 7 actin monomers

Troponin T binds to TM at head-to-tail junction


Composition & Structure of Thick FilamentsMyosin - 2 heavy chains, 4 light chains Heavy chains - 230 kD each Light chains - 2 pairs of different 20 kD chains

The "heads" of heavy chains have ATPase activity and hydrolysis here drives contraction

Light chains are homologous to calmodulin and also to TnC

See structure of heads in Figure 16.16


Myosin Cartoon EM S1 myosin head structure


Repeating Structural Elements Are the Secret of Myosin’s Coiled Coils 7-residue, 28-residue and 196-residue repeats are responsible for the organization of thick filaments

Residues 1 and 4 (a and d) of the seven-residue repeat are hydrophobic; residues 2,3 and 6 (b, c and f) are ionic

This repeating pattern favors formation of coiled coil of tails. (With 3.6 - NOT 3.5 - residues per turn, -helices will coil!)


Axial view (fig. 16.17)Myosin tail: 2-stranded -helical coiled coil


More Myosin Repeats!

28-residue repeat (4 x 7) consists of distinct patterns of alternating side-chain charge (+ vs -), and these regions pack with regions of opposite charge on adjacent myosins to stabilize the filament

196-residue repeat (7 x 28) pattern also contributes to packing and stability of filaments


Myosin packing Adjoining molecules offset by ~ 14 nm Corresponds to 98 residues of coiled coil


Associated proteins of Muscle -Actinin, a protein that contains

several repeat units, forms dimers and contains actin-binding regions, and is analogous in some ways to dystrophin

Dystrophin is the protein product of the first gene to be associated with muscular dystrophy - actually Duchennes MD

See the box on pages 524-525


DystrophinNew Developments!

Dystrophin is part of a large complex of glycoproteins that bridges the inner cytoskeleton (actin filaments) and the extracellular matrix (via a protein called laminin)

Two subcomplexes: dystroglycan and sarcoglycan

Defects in these proteins have now been linked to other forms of muscular dystrophy



Nick Menhart:BCPS faculty member specializing in dystrophin research


Dystrophin, actinin,spectrin

Characteristic 3-helix regions


Spectrin-repeat structure These characteristic 3-helix elements are found in actinin, spectrin, dystrophin



Spectrin repeatPDB 1AJ3NMR12.8 kDa


Model for complex

Actin-dystrophin-glycoprotein complex

Dystrophin forms tetramers of antiparallel monomers


The Dystrophin Complex

Links to disease -Dystroglycan - extracellular, binds to merosin (a component of laminin) - mutation in merosin linked to severe congenital muscular dystrophy

-Dystroglycan - transmembrane protein that binds dystrophin inside

Sarcoglycan complex - , , - all transmembrane - defects linked to limb-girdle MD and autosomal recessive MD


The Sliding Filament ModelMany contributors!

Hugh Huxley and Jean Hanson Andrew Huxley and Ralph Niedergerke

Albert Szent-Györgyi showed that actin and myosin associate (actomyosin complex)

Sarcomeres decrease length during contraction (see Figure 16.19)

Szent-Gyorgyi also showed that ATP causes the actomyosin complex to dissociate





Hugh Huxley

Albert Szent-Györgyi


Sliding filaments Decrease in sarcomere length happens because

of decreases in width of I band and H zone No change in width of A band Thin & thick filaments are sliding past one another


The Contraction Cycle

Study Figure 16.20! Cross-bridge formation is followed by power stroke with ADP and Pi release

ATP binding causes dissociation of myosin heads and reorientation of myosin head

Details of the conformational change in the myosin heads are coming to light!

Evidence now exists for a movement of at least 35 Å in the conformation change between the ADP-bound state and ADP-free state


Mechanism

Fig. 16.20


Actin-myosin interaction Ribbon- and space-filling representations





Ivan Rayment

Hazel Holden


Similarities in Motor Proteins Initial events of myosin and kinesin action are similar

But the conformational changes that induce movement are different in myosins, kinesins, and dyneins


Myosin & kinesin motor domains

Relay helix moves back and forth like a piston


Intramolecular communication & conformational changes Myosin and kinesin:ATP hydrolysis conformational change that gets communicated to track-binding site

Dynein: not well understood; involves AAA ATPases


Muscle Contraction Is Regulated by Ca2+Ca2+ Channels and Pumps

Release of Ca2+ from the SR triggers contraction

Reuptake of Ca2+ into SR relaxes muscle

So how is calcium released in response to nerve impulses?

Answer has come from studies of antagonist molecules that block Ca2+ channel activity


Ca2+ triggers contraction Release of Ca2+ through voltage- or Ca2+-sensitive channel activates contraction

Pumps induce relaxation


Dihydropyridine ReceptorIn t-tubules of heart and skeletal

muscle Nifedipine and other DHP-like molecules bind to the "DHP receptor" in t-tubules

In heart, DHP receptor is a voltage-gated Ca2+ channel

In skeletal muscle, DHP receptor is apparently a voltage-sensing protein and probably undergoes voltage-dependent conformational changes


Ryanodine ReceptorThe "foot structure" in terminal

cisternae of SR Foot structure is a Ca2+ channel of unusual design

Conformation change or Ca2+ -channel activity of DHP receptor apparently gates the ryanodine receptor, opening and closing Ca2+ channels

Many details are yet to be elucidated!


Ryanodine Receptor

Courtesy BBRI




Muscle Contraction Is Regulated by Ca

2+

Tropomyosin and troponins mediate the effects of Ca2+

See Figure 16.24 In absence of Ca2+, TnI binds to actin to keep myosin off

TnI and TnT interact with tropomyosin to keep tropomyosin away from the groove between adjacent actins

But Ca2+ binding changes all this!


Ca 2+ Turns on Contraction

Binding of Ca2+ to TnC increases binding of TnC to TnI, simultaneously decreasing the interaction of TnI with actin

This allows tropomyosin to slide down into the actin groove, exposing myosin-binding sites on actin and initiating contraction

Since troponin complex interacts only with every 7th actin, the conformational changes must be cooperative


Thin & thick filaments Changes that happen when Ca2+ binds to troponin C

Fig. 16.24


Binding of Ca 2+ to Troponin C Four sites for Ca2+ on TnC - I, II, III

and IV Sites I & II are N-terminal; III and IV on C term

Sites III and IV usually have Ca2+ bound Sites I and II are empty in resting state Rise of Ca2+ levels fills sites I and II Conformation change facilitates binding of TnC to TnI


2 views of troponin C Ribbon Molecular graphic

Fig. 16.25


Smooth Muscle Contraction

No troponin complex in smooth muscle

In smooth muscle, Ca2+ activates myosin light chain kinase (MLCK) which phosphorylates LC2, the regulatory light chain of myosin

Ca2+ effect is via calmodulin - a cousin of Troponin C


Effect of hormones on smooth muscle Hormones regulate contraction - epinephrine, a smooth muscle relaxer, activates adenylyl cyclase, making cAMP, which activates protein kinase, which phosphorylates MLCK, inactivating MLCK and relaxing muscle


Smooth Muscle Effectors

Useful drugs Epinephrine (as Primatene) is an over-the-counter asthma drug, but it acts on heart as well as on lungs - a possible problem!

Albuterol is a more selective smooth muscle relaxer and acts more on lungs than heart

Albuterol is used to prevent premature labor

Oxytocin (pitocin) stimulates contraction of uterine smooth muscle, inducing labor


Oxytocin structure P.532

12/02/2008biochemistry: muscles muscle contraction andy howard introductory biochemistry 2 december...

Documents

muscles page

sr slide

irving slide

myofibril slide

nm actin filaments

troponin c smooth muscle

troponin c tnc slide

muscle movement