BIOL 201 Lecture 23 Myosin and Muscle

Sliding filament theory

Our muscles are made up of filaments that move by sliding past one another When our muscles are relaxed, the muscle cells are stretched When the muscles are contracted, then the muscle cells are fat and short When they are relaxes, muscle cells are long and slender The reason this occurs is b/c actin filaments within them are being contracted by these myosin II bundles i.e. they are pulled inward and slide over the myosin, reducing the fluid filled space b/w the myosin and Z disk, which reduces the overall length of the sarcomere Z disks are shown 120% apart and as myosin contracts, they contract to 60% of the relaxed length This is the basis for the sliding filament theory Sarcomeres are the basic unit of the "sliding filament theory"

Muscles face 3 design challenges

1. Prevent continuous contraction 2. Activate contraction 3. Freeze the structure of the sarcomere There are 3 major challenges muscles face They need to prevent muscles from continuously contracting How do cells block contraction? How do they activate contraction as well if they can stop it? We also need to be able to freeze the structure of the sacromere Actin wants to polymerize so how do we maintain actin filaments of the same size? Actin wants to grow! You can't take away the actin because then it will shrink Need some sort of optimum concentration of actin where no treadmilling is occuring

Rigor mortis

Rigor mortis is the stiffening of the muscles of our body after we die Bodies stiffen within ½ hour This artist created a doll demonstrating rigor mortis We can understand rigor mortis as the failure of myosin to detach from the actin filaments because there is no more ATP Your body isn't producing ATP anymore after you die The myosin head will stay attached to the actin and then the muscles can no longer be moved When we eat meat, what you're eating is actin and myosin Protein in meat is mainly actin The meat industry considers muscles meat when it is in rigor mortis state This is why you can't flex a chicken's wings It is in rigor mortis

Sarcomeres

Sarcomeres are tightly packed arrays of actin filaments and myosin filaments The cross section of a sarcomere is shown The smaller dots are the actin filament bundles and the bigger dots are the myosin filaments These actin and myosin bundles are packed in the sarcomere Sarcomeres undergo rapid contraction upon muscle stimulation Sarcomeres are composed of long, fibrous proteins that slide past each other when the muscles contract and relax. Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin. The myosin head also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions.

Muscle Contraction

Contraction is fast Contraction from fully relaxed state to fully contracted only takes 50 miliseconds Type II fibers are fast twitch fibers Type I fibers are not as fast but can bear stronger loads This explains why some people may be better at certain athletic activities such as running or weightlifting Training in these activities cultivate these fibers as well Ex. if you're weightlifting, you'll develop more Type I fibers The first phase is 5-10 ms "latent period". This is amount of time it takes for the signal generated by your brain to travel down a neuronal axon and initiate muscle contraction The second phase is only a 40-45 ms contraction phase where sarcomeres go from fully elongated to fully contracted 5-10 miliseconds is the amount of time it takes for the signal form the brain to start contraction There is a lot of significant biochemistry occurring during this process

The "Power stroke"

We now understand the power stroke at very high resolution The relaxed state is more straightened while the head cocked is more vertical The length b/w the myosin head and where it connects into the myosin II bundle is known as the neck domain The length of this neck domain is important If it were longer, the power stroke would be bigger Scientists have actually replicated this where the neck domain is acting as a lever arm The longer the neck domain, the greater the force This is similar to a crow bar They lengthen the neck domain and add the myosin head The longer the neck, the faster the myosin translocated the actin filament Lever arms amplify force even at the microscopic level!

The Crossbridge Cycle

We will now look at the process of myosin contraction also known as the Crossbridge Cycle aka the ATPase cycle of myosin It involves ATP hydrolysis 1 ATP hydrolysis event by the myosin motor leads to one kick by the myosin to the actin filament We will start where the myosin head has just completed its cycle It detaches from the actin filament by binding to ATP Myosin will bind to ATP and pop off the actin filament Once myosin has a hold of ATP, it will rapidly and reversibly hydrolyze it The myosin is pushed into a strained or cocked position once ATP hydrolysis occurs All of the myosin's atoms are highly strained in this state and thus this is not favorable In this ADP/Pi state, the myosin is now positioned above an actin subunit This myosin really likes to bind actin in this ADP state The myosin head will bind the actin filament in the ADP state This triggers changes in the nucleotide pocket, allowing the release of products Phosphate will be released first This will trigger all the energy from the strained position being released This energy works to move the actin filament in a power stroke The myosin will relax then All the strained energy is released and this pushes the actin filament left ADP is released and ATP is bound The myosin is released from the actin The affinity of myosin for the filament changes during this cycle The myosin also transitions b/w two states: a relaxed state and a strained state This whole process takes 10 miliseconds On average, each myosin head will go through this cycle 4 times when your muscles are contracting From the time the myosin binds to the filament to the time it releases, this time is very short The amount of time it is bound to the actin filament is called the duty cycle and usually only 1-2 miliseconds of the 10 miliseconds For muscle myosin, this duty cycle is only a milisecond or two This is because we don't want contraction occuring in the opposite direction We don't want two myosins contracting in opposite directions at the same time It is very important that the actin not hinder contraction This is why the duty cycle is so short

The structure of sarcomeres is set by nebulin, titin, and capping proteins

We'll look at number 3 first Sarcomere structure is set by 3 proteins: nebulin, titin and capping proteins Capping proteins are tropomodulin and cap Z CapZ caps the positive end of actin and tropomodulin caps the minus end Titin is one of the largest proteins produced and is represented by the orange line. It extends from one Z disk to the other and sets the intrinsic spacing of the Z disks It stretches from one Z disk to the other Nebulin is protein that acts as a ruler for the thin filaments Nebulin extends out of one Z disk and extends out to the tropomodulin The length of the actin filament is set because nebulin can bring capping proteins, tropomodulin and CapZ, to the positive and minus ends

Motor Neurons

Motor neurons are the neurons that activate muscle contraction Motor Neurons transmit signals from the brain that activate muscle contraction Motor neurons cause an increase in calcium concentrations The stimulation by a motor neuron causes a rapid spike in the calcium concentrations The membrane of muscle cells become depolarized and triggers calcium concentration increase This occurs because these muscles have sarcoplasmic reticulum, which store calcium Muscle contraction is stimulated by the presence of calcium The rate of ATP hydrolysis is thus related to calcium concentrations How does calcium pull trypomyson though out of the way?

Muscles

Muscles are made of billions of these contractile muscles If you look at these muscles in greater detail, you find first bundles of muscle fibers and these bundles are made up of several multinucleated muscle cells Inside each cell you find myofibril Myofibril is made up of repeated units of sarcomeres These sarcomeres made up of actin filaments Sarcomeres are made of contractile arrays of interdigitating actin and myosin Myosins were once known as thick filaments The actin filaments were called thin filaments Muscles can generate both massive force and refined movement Muscle contraction is more than just lifting It is also behind speech movement, which is very refined

Myosin II

Myosin II are found in bundles in our muscles. If all the myosin heads on the left pull in and the myosin heads on the right pull in, anti-parallel actin arrays that are interacting with this myosin bundle will be pulled inward This means that muscle is contracted The array shortens compared to the relaxed position This is the basic mechanism of contraction that underlies pulling up the cell's foot during cell migration as well as what we see in muscle contractio n

Myosin Power Stroke

Myosin binds to actin and undergoes a conformational change in response to the release of phosphate, causing the actin filament to translocate.

Myosin

Myosin has a head domain which has a binding site for actin Myosin is a motor protein, meaning it can convert chemical energy into mechanical work or force These are the world's smallest engines They do the same thing a car engine does Myosin contains a motor domain or head domain that has an actin-binding site and nucleotide binding site which binds to ATP Myosin also has a region that from emerges from the domain, which will couple into larger oligomeric assemblies A myosin head is shown attached to an actin filament The release of phosphate will translocate the actin filament because of the conformational change in the myosin Myosin-II forms bundles that pull actin filaments inward We discussed how myosin bundles contract with actin This actin array shortens This is the basic mechanism of contraction It is how a cell's foot moves and how our muscles move

Tropomyosin

Tropomyosin blocks the binding site for myosin on the actin filament So how do you prevent continuous contraction? The blocking of continuous contractions comes from proteins that wrap around the actin filament called tropomyosin Tropomyosin is shown in yellow and coils around the actin It blocks the myosin head from binding to the actin We then need to move tropomyosin out of the way if we want a contraction in the muscles

How does calcium pull trypomyson though out of the way?

Troponin binds calcium and pulls tropomyosin out of the way Troponin is a calcium binding protein which binds to calcium and pulls tropomyosin off of the myosin binding site Calcium binds to troponin and pulls it into a different conformation This pulls the trypomyosin out of the myosin binding site This is what happens when your motor neurons tell your muscles to contract