Muscle Fiber Composition and Function: Actin Filaments, Myosin, and Proteins at the Z-Disk, Study notes of Biochemistry

Download Muscle Fiber Composition and Function: Actin Filaments, Myosin, and Proteins at the Z-Disk and more Study notes Biochemistry in PDF only on Docsity! Exercise/Physiology Test 3 I. Structural Aspects of Skeletal Muscle a. Know what general proteins that comprise the thick and thin filaments. i. Thick Filament Proteins 1. Myosin: contractile protein that splits ATP and is responsible for the “power stroke” of the myosin head. 2. C-Protein : structural protein that holds the myosin tails in a correct spatial arrangement; may hold H protein of an adjacent thick filament at an even distance during force generation; may also control the number of myosin molecules in a thick filament 3. M-protein : helps hold the thick filament in place (part of M-line) 4. Myomesin : provides strong anchoring point for the protein Titin. 5. M-CK - provides ATP from phosphocreatine; located proximal to the myosin heads. (part of the M-line). 6. α- actin: attaches actin filaments together at the Z-disk actin: actin: attaches actin filaments together at the Z-disk attaches actin: attaches actin filaments together at the Z-disk actin actin: attaches actin filaments together at the Z-disk filaments actin: attaches actin filaments together at the Z-disk together actin: attaches actin filaments together at the Z-disk at actin: attaches actin filaments together at the Z-disk the actin: attaches actin filaments together at the Z-disk Z-disk actin: attaches actin filaments together at the Z-disk 7. Desmin, actin: attaches actin filaments together at the Z-disk vimentin, actin: attaches actin filaments together at the Z-disk synemin actin: attaches actin filaments together at the Z-disk actin: attaches actin filaments together at the Z-disk : actin: attaches actin filaments together at the Z-disk link actin: attaches actin filaments together at the Z-disk Z-disks actin: attaches actin filaments together at the Z-disk of actin: attaches actin filaments together at the Z-disk adjacent actin: attaches actin filaments together at the Z-disk myofibrils actin: attaches actin filaments together at the Z-disk together 8. Spectral actin: attaches actin filaments together at the Z-disk and actin: attaches actin filaments together at the Z-disk dystrophin actin: attaches actin filaments together at the Z-disk actin: attaches actin filaments together at the Z-disk : actin: attaches actin filaments together at the Z-disk have actin: attaches actin filaments together at the Z-disk structural actin: attaches actin filaments together at the Z-disk and actin: attaches actin filaments together at the Z-disk functional actin: attaches actin filaments together at the Z-disk roles actin: attaches actin filaments together at the Z-disk as actin: attaches actin filaments together at the Z-disk sarcolemma actin: attaches actin filaments together at the Z-disk proteins actin: attaches actin filaments together at the Z-disk 9. Titin actin: attaches actin filaments together at the Z-disk actin: attaches actin filaments together at the Z-disk : actin: attaches actin filaments together at the Z-disk Helps actin: attaches actin filaments together at the Z-disk keep actin: attaches actin filaments together at the Z-disk the actin: attaches actin filaments together at the Z-disk thick actin: attaches actin filaments together at the Z-disk filament actin: attaches actin filaments together at the Z-disk centered actin: attaches actin filaments together at the Z-disk between actin: attaches actin filaments together at the Z-disk the actin: attaches actin filaments together at the Z-disk two actin: attaches actin filaments together at the Z-disk Z- lines actin: attaches actin filaments together at the Z-disk during actin: attaches actin filaments together at the Z-disk contraction actin: attaches actin filaments together at the Z-disk by actin: attaches actin filaments together at the Z-disk linking actin: attaches actin filaments together at the Z-disk them. actin: attaches actin filaments together at the Z-disk ii. Thin Filament Proteins 1. Actin : main contractile protein interacting with myosin during excitation-contraction coupling 2. Tropomyosin: Regulatory, rope-like protein that extends over the entire length of F-actin. Under resting conditions, it blocks the myosin binding sites on actin. 3. Troponin: Found in every 7th G-actin along the thin filament. Three subunits: 1. TnI- inhibitory subunit: inhibits movement of tropomyosin by positioning tropomyosin on the actin binding site. 2. TnT- tropomyosin-binding subunit: binds loosely onto tropomyosin to prevent it from moving off of actin 3. TnC- calcium binding subunit: contains four binding sites for calcium; two of them have high affinity and at rest Mg2+ binds to them. Two of the binding sites have low affinity for Ca2+ and they are empty at rest. (Cardiac muscle only has 3 total ions). Activation: when the intracellular calcium level rises to a critical level, four calcium ions bind to the TnC, thereby causing the entire subunit configuration to change. TnT then binds tightly to tropomyosin and the entire troponin protein physically moves tropomyosin to expose the myosin-binding sites on actin. Once the myosin-binding site is exposed, S-1 sub-fragment of myosin can insert (creating cross bridge). b. Know what the sarcolemma and sarcoplasm are. i. The sarcolemma surrounds the muscle fiber beneath the endomysium and encloses the fiber’s cellular contents. It is comprised of both the plasma and basement membranes. 1. The basement membrane is the outermost covering; collection of glycoproteins and collagen network that is freely permeable to proteins, solutes, and other metabolites. 2. The plasma membrane is the true cell boundary that is found just beneath the basement membrane. The plasma membrane is made up of a lipid bilayer and is much more selective to ions, solutes, and substrates. Functions: - Maintains the proper acid-base balance, allowing it to contract repeatedly during exercise - Involved in propagating an action potential that will lead to muscle contraction (conducts wave of depolarization over each muscle fiber) - Transports metabolites from the blood in the capillaries to the center cytosol of the muscle fiber - Serves as an elaborate region of functional folds at the neuromuscular junction - Contains caveolae which provide additional length during fiber stretching; allows lengthening to occur without damaging the plasma membrane - Insulates the fibers ii. The sarcoplasm: interior of the plasma membrane; rich in soluble proteins, myofilaments, and true myonuclei; as well as high-energy intermediates (ATP, PC), substrates (glycogen and lipids), enzymes of metabolism, mitochondrial proteins, ribosomes for protein synthesis, cytosolic proteins, and so on. c. Know what the t-tubules and sarcoplasmic reticulum are. i. Sarcoplasmic reticulum : network of tubular channels and vesicles that provide the structural integrity to the cell. Allows for the depolarization waves to spread rapidly from outer cell to inner cell. Stores calcium ions. ii. T-Tubule system : found within the SR; provides for the spreading of depolarization waves; contains pumps that take up calcium from the fiber’s sarcoplasm, creating a calcium gradient between the SR(higher) and the sarcoplasm surrounding the filaments (lower). d. What is a sarcomere and what are its boundaries? A sarcomere is the strongest contractile unit of a skeletal myofiber. The sarcomere length is the distance from one Z-disk to the next (optimal length-2.5 μm). The optimal length is directly proportional to the capacity for force generation (doesn’t have high force capacity if it’s too small or too large- length/tension relationship).  A-band: dark band  I-band: isotropic band  H-zone: central region of the A-band where there is no thick and thin filament overlap; H-zone is bisected by the M-line  M-line: composed of proteins that keep the sarcomere in proper spatial orientation as it lengthens and shortens  Z-disks: most dense troponin prevents tropomyosin from moving off of the myosin-binding site on actin so that cross-bridge formation cannot form. c. Describe the chemical and mechanical steps in cross-bridge cycle and explain how the cross-bridge cycle results in shortening of the muscle: sliding filament hypothesis and Hypothetical scheme of cross-bridge cycle etc. i. Chemical/mechanical steps : 1. In resting state, actin and energized myosin (ADP + Pi) cannot interact because of the blocking effect of tropomyosin (regulated by troponin). Upon release of calcium from the SR, energized myosin binds to actin. 2. Tension is developed and movement “power stroke” occurs with the release of ADP and Pi as energy. This leaves the myosin head open for binding. 3. Dissociation of actin and myosin requires the presence of ATP to bind to the myosin head so that it can pump calcium back into the SR. 4. ATP is then hydrolyzed and myosin returns to its energized resting state. ii. Sliding filament model : The thick and thin filaments move in relation to each other (without changing length), resulting in change in sarcomere and therefore muscle length. The myosin cross-bridges cyclically attach, rotate, and detatch from the actin filaments with energy from ATP hydrolysis providing the motor to drive fiber shortening. The sarcomere’s zones and bands change, producing a force at the Z-band (The H-zones disappear and the I-band becomes very narrow as the sarcomere shortens; A band stays the same.) d. Principles, theories, and relationsihips: eg. Size principle, length-tension relationship and etc. i. Size Principle : As muscle force requirements increase, motor neurons are recruited with progressively larger axons, expressing a recruitment order to produce muscle action. The selective recruitment and firing pattern of fast- twitch and slow-twitch motor units that control stabilizing regions provide the mechanism to produce the desired coordinated response. Slow-twitch motor units with lower thresholds for activation are recruited during light-moderate effort (jogging, cycling, swimming). More rapid, powerful movements activate first type IIa and then type IIx (weight lifting). Type I type IIa type IIb ii. Length-tension relationship : The tension developed is proportional to the degree of overlap between actin and myosin filaments and hence to the number of active cross-bridges. The optimum length of the sarcomere (2-2.2 μm) corresponds to the greatest overlap and the highest active tension. 1. When the muscle fiber is stretched further than this optimal length, there was no overlap between the myosin and actin filaments so no active tension was developed (beyond 3.75μm). 2. When the sarcomere length was shorter (1.67μm), thin filaments from opposite ends of the sarcomere interfere with each other. When the sarcomere length is decreased to the length of the thick filament, there is no developed force. iii. Force-Velocity : the velocity of muscle contraction is inversely proportional to the load. Large forces cannot be exerted in very rapid movements. The greatest velocities are attained under conditions of low loading. This occurs because the force generated by the muscle depends on the total number of cross-bridges attached before the power stroke. As velocity increases, the filaments slide past each other faster and faster so the filaments are attached to each other for less time. This causes the force to decrease. Conversely, as velocity decreases, the filaments slide past each other at a slower rate and allow for more cross-bridge attachments. This generates a greater force when the power stroke occurs. 1. Type I fibers have a lower velocity and force when compared with Type II fibers. 2. Training can increase both velocity and force. (Force greater). e. Know excitation-contraction (EC) coupling: be able to describe the sequences involved in EC-coupling-muscle contraction from release of ACH from the motor neuron all the way until relaxation. i. Features : 1. EC-coupling is the process of converting an electrical stimulus (Action Potential) to mechanical response (contraction). 2. Fundamental to muscle physiology- can be dysregulated in many disease conditions ii. Sequences : 1. An action potential originates in the CNS and travels to a α-motor neuron which then transmits the AP down its own axon. The AP then activates voltage-dependent Ca2+ channels on the axon and Calcium rushes in. 2. The increase in calcium causes synaptic vesicles containing the ACh to fuse with the plasma membrane at the active zone. The vesicles then release ACh into the synaptic cleft via exocytosis and ACh diffuses across the synapse. It then binds to and activates nicotinic ACh receptor on the motor end plate. 1. Half of the ACh molecules are hydrolyzed by acetylcholinesterase 3. Binding of ACh to α-subunit of ACh receptor opens the cation gated chanel and allows Na+ rush in and K+ to trickle out. The muscle fiber membrane then becomes more positively charged, triggering an AP. 4. The AP spreads through the muscle fiber’s network of T-tubules, depolarizing the inner portion of the muscle fiber. This stimulates Ca2+ release. 5. Calcium binds to Troponin C subunit which then activates it to bind to tropomyosin which moves the blocking effect off of the myosin-binding site on actin. Myosin is then able to bind to the actin filament and release its power stroke to shorten the sarcomere. These contraction steps repeat as long as ATP is available and Ca2+ is available in the thin filament. 6. Relaxation: The myosin ceases binding to the thin filament and the contractions come to a halt. Meanwhile, Ca2+ is actively pumped back in the SR to maintain sarcoplasmic calcium concentrations. The active pumping of calcium ions back into the SR creates a deficiency in the fluid around the myofibrils. When calcium concentrations become depleted, tropomyosin changes conformation so that it can block the binding site on actin again. *** Duration and intensity of active state depend on calcium concentrations around the contractile filaments. f. The mechanical coupling hypothesis. 1. Release of calcium from SR during EC-coupling is known to be a result of coordinated functional interaction of muscle surface membrane calcium channels (DHPRs) with SR calcium release channels (RyR1s) in skeletal muscle. 2. Arrival of AP activates a voltage sensor, DHPR, on the external membrane by changing the conformation of it. (Dispositions the 4 subunits of DHPR that link to 4 subunits of RyRs). This causes the opening of 4 subunits of RyR1, thereby resulting in a release of Ca2+ into the cytosol of the skeletal muscle. 1. DHPR- voltage-dependent calcium channel found in the T- tubules of muscles; associates with RyRs of the SR to induce calcium release and thus muscle contraction 2. RyRs: Ryanodine receptros. SR Ca2+ release channel that is essential for muscle contraction; activation occurs by coupling to the DHP channel. Binding of ryanodine modifies the RyR1’s conductance and gating by causing a partial opening state at low concentration and a fully closed channel at high concentration. (triggering one will trigger all RyR1 receptors) 3. By triggering one RyR1 channel, it could activate all associated RyR1 channels in a junction. Coordinated activation would ensure the speed of skeletal muscle contraction. Correlates to the all-or-none twitch in skeletal muscle. g. Actions of Calcium ions in skeletal muscle and its calcium pumps. i. Functions of calcium ions: 1. Initiating the contractile process in muscle fibers 2. Acting as a second messenger: releasing ACh from motor nerve terminals 3. In damaged plasmalemma, calcium ions will flow down their electrochemical gradient and disrupt the structure and function of the cytoplasmic contents. ii. Calcium Pumps (2): both move calcium ions against large gradients; both are stimulated by the presence of Mg2+ ions; both require hydrolysis of ATP for energy which has the ability to transport 2 Ca2+ ions across the membrane. 1. Surface membrane pump (SMP): larger than SRP; requires Ca2+:calmodulin binding site in order for the pump to be activated. 2. Sarcoplasmic reticulum pump (SRP): responds to Ca2+ ions directly. 1. Each pump has a low capacity for transporting ions, thus a high density of pumps in the SR membrane is necessary to bring down calcium concentration in the cytosol quickly. 2. With each cycle of pump, 2 Ca2+ ions are transported into the SR in exchange for 2 K+ ions. Unequal exchange causes the cytosol near the SR to become increasingly negative. *The recommendations for most middle-aged and older adults focus on maintaining muscle and bone mass and muscular strength and muscular endurance to enhance the overall health. ii. The muscle is capable of these actions because of 4 factors: 1. Specificity: the underlying attributes of a skill are specific to that skill and generally have low transfer. The greatest gains will occur by performing the specific task you want to be improved 2. Accommodation: the response of a biological object to a given stimulus decreases over time. If you do the same thing each workout, your body will stop adapting. 3. Overload principle: adaptation only takes place if the magnitude of training load is above habitual level. Most basic way would be to increase the weight that you lift. 4. Variety: your body needs variety to adapt either qualitatively or quantitatively. IV. Muscular Diseases and Conditions a. Know the general etiology of muscular dystrophy. i. Group of genetic hereditary muscle diseases that cause progressive muscle weakness. Characterized by progressive skeletal muscle wasting weakness), defects in muscle proteins, and death of muscle cells and tissue. 1. Symptoms : mostly affects children 1. Poor balance 2. Frequent falls 3. Walking difficulty 4. Drooping eyelids 5. Affects on cardiac and smooth muscle cells (heart/organ dysfunction) 2. 9 diseases : Duchenne, Becker, Limb girdle, congenital, fascioscapulohumeral, myotonic, oculopharyngeal, distal, and Emery- Dreifuss (NIH) 1. Duchene Muscular Dystrophy (DMD): 1/3300 births. Caused by mutations in the gene which encodes dystrophin, causing it to be absent. Onsets at age 2-6.  Dystropin: vital part of a protein complex that anchors the sarcomere to the myofibril membrane, keeping the muscle integrity. Acts as an anchor protein for the actin filament. Lack of dystropin causes contraction induced injury in their muscles because they go into degeneration rapidly. The degenerated muscles are replaced by connective tissue which virtually has no function. - Can affect young males due to its X-linked recessive inheritance pattern (2/3 come from mother; 1/3 caused by mutations in the genes of the egg or embryo) - Symptoms: affects pelvis, upper arms, and upper legs. Involves all voluntary muscles; no cure- survival beyond 20 is rare. 2. Becker’s Muscular Dystrophy (BMD): 1/18,430 births. Dystrophin is of abnormal weight and shape; onset 2-16 years Symptoms are less severe than DMD; survive until middle age. Eventually those with MD lose the ability to breathe because the diaphragm is a skeletal muscle that gets replaced with connective tissue. 3. Diagnosis : muscle biopsy, DNA blood test, physical examination/medical history, loss of muscle mass 4. Treatment : No specific treatment; inactivity can worsen the disease. Daily physical therapy and orthopedic instruments may be helpful. Muscle weakness may be attenuated by regular, low intensity exercise. 1. Exercise recommendations: Studies addressing physical training were successfully carried out in mice with MD. Researchers were able to define thresholds and intensity, frequency and duration to minimize the dystrophic process and improve muscle function. This was possible because they were able to define the disease and exercise response mechanisms. - Human testing has not been carried out because the disease and exercise response mechanisms are undefined and the risk for exacerbating the dystrophic process is too high. b. Know the general etiology and exercise treatment for sarcopenia. i. Definition: Sarcopenia refers to the degenerative loss of skeletal muscle mass and strength associated with aging. This leads to multiple problems regarding everyday lifestyle and functions related to mobility. ii. Trends: 1. The general trend is a decrease in the size of type II fibers (both type IIa and type IIx) and an increase in type I fibers to compensate for this loss. This correlates to elderly people losing power and velocity of muscular contractility; this correlates to why elderly people move slower and are more susceptible to fall-related injuries. When the elderly person falls, they don’t have the power to respond quickly enough. 2. Another trend seen is a grouping affect of type I muscle fibers. This is necessary because neurons tend to die with old age since they aren’t being used. In order to accommodate for this, the type I muscle fibers group together so that they can be innervated by the same motor unit. This produces more synchronous muscle velocities which help with a smoother mobility. Elderly people without this feature can become very shaky because their muscle fibers are all contracting at different velocities. 3. Another trend lies between males and females. Elderly females are the most drastically affected by sacropenia. This could have to do with different muscle mass ratios between males and females, which aging seems to magnify. Young males contain the optimal muscle fiber content. iii. Exercise treatment : 1. Load-Mediated Myofiber Hypertrophy: there was a slight increase in Type II muscle fibers for the elderly male population with 16 weeks of load-mediated hypertrophy training. 2. Endurance: Cross-sectional areas of fibers for elderly sprinters changed mostly in the content of type IIx muscle fibers. The concentration of type I muscle fibers increased to compensate for this type IIx fiber loss. iv. Application : people are recommended to stay active and trained so that when aging occurs, less type II muscle fibers are lost and they are able to use power functions of their muscles. This would help reduce the physical dangers of aging individuals.