Biochem Chapter 7 Muscle contraction

Who discovered the sliding filament model?

Hugh Huxley

What is myosin composed of?

6 polypeptide chains: 2 heavy chains 2 light chains and essential and regulatory light chains (ELC and RLC)

How many actin subunits does tropomyosin contact?

7 subunits

What are the regions of greater electron density?

A bands

What does TnI do?

Binds to actin

What does TnT do?

Binds to tropomyosin

What does TnC do?

Ca2+ binding protein

What triggers muscle contraction?

Ca2+ released from the sarcoplasmic reticulum

What does tropomodulin do?

Caps (-) end of thin filament to prevent further actin polymerization and depolymerization

What role does actin play in nonmuscle cells?

Cell shape, cell division, endocytosis, and organelle transport

Components of troponin?

TnC, TnI, and TnT

What are the other 2 components of thin filament?

Tropomyosin and Troponin

What causes myosin binding to actin sites to block?

Troponin-tropomyosin complex, muscle relaxes

What are additional muscle proteins that help organize the structure?

a-actinin, titin, nebulin, tropomodulin, capZ, Myomensin, M-protein, and Dystrophin

How are microfilaments observed?

immunofluorescence microscopy

What is the C-terminal half of the heavy chain?

Fibrous a-helical tail, forming a left-handed coiled coil

What is treadmilling driven by?

Free energy of ATP hydrolysis

Where is the sarcomere?

From z disc to z disc

What is tropomyosin?

Homodimer, 284-residue a-helical subunits wrap around each other to form parallel coiled coil. Waraps around actin head to tail

How does myosin form the thick filament?

Hundreds of myosin molecules pack end to end in a staggered way with globular head projecting to the sides

What are the regions of lesser electron density?

I bands

What are the forms of actin?

In monomeric form = G-actin (globular) When polymerized = F-actin (fibrous)

Where are the thick filaments?

In the A band

Where are the thin filaments?

In the I band

What is the sliding filament model?

Interdigitated thin and thick filaments move past each other to shorten the overall length of the sarcomere

What does the tropomyosin-troponin complex regulate?

Muscle contraction by controlling access of myosin heads to actin binding sites

What two proteins are associated with the M disk?

Myomensin and M-protein

What is a motor protein?

Protein that converts ATP hydrolysis energy to mechanical energy of movement. Myosin is an example of this

Why is voluntary muscle striated?

Regions of greater and lesser electron density

What is the sarcomere?

Repeating unit of myofibril, containing thick/thin filaments that slide during muscle contraction

What does nebulin do?

Sets the length of thin filament by acting as a template for actin polymerization

What does CapZ (B-actinin) do?

Caps (+) end of F-actin

What is thick filament composed of?

Mainly myosin

What is microfilament treadmilling?

Microfilaments are constanly depolymerizing and repolymerizing. Subunits add to (+) end and dissociate at the (-) end at a steady rate

What are the steps of force generation?

1. ATP binds to myosin head -->opens myosin actin binding site, releases bound actin 2. Myosin active site closes around ATP. Hydrolysis to ADP +Pi cocks the myosin head 3. Myosin head binds weakly to actin monomer 4. Myosin releases Pi, causing actin-binding site to close and increases actin affinity 5. Power stroke bring conformational shift 6. ADP is released, completing the cycle

What binding sites does actin have?

ATP, Ca2+/Mg2+

What is the myosin head?

ATPase (ATP-hydrolyzing enzyme)

What are thin filaments composed of?

Actin

What does dystrophin do?

Anchors F-actin to extracellular matrix and protects plasma membrane from tears and stress

What does a-actinin do?

Attaches oppositely oriented thin filaments to Z disk. Located in Z disk interior

What are the characteristics of actin?

Double stranded helix of subunits contacting 4 others. Each subunit has same head-to-tail orientation, giving (-) and (+) end

F-actin binding myosin heads

Each F-actin monomeric unit can bind a single myosin head (ion pairing or hydrophobic interactions)

What are the characteristics of the myosin head?

It is the N-terminal half of myosin heavy chain, has globular head

What does titin do?

Keeps thick filament centered on sarcomere, compresses and shortens sarcomere during contraction and resists sarcomere extension past starting point

What do voluntary muscles consist of?

Long multinucleated cells (muscle fibers) and bundles of myofibrils

What are microfilaments?

Nonmuscle actin forms, cytoskeletal element

Where is the (+) end?

Opposite end, that binds with Z disc

What appearance does voluntary muscle have?

Striated appearance

What is bound to the myosin head?

Subunit of ELC and RLC

What are crawling cells?

The directional growth of actin filaments that exerts force against the plasma membrane, allowing a cell to extend its cytoplasm, trailing edge releases contact with surface to move forward

Where is the (-) end?

The end opening toward the nucleotide binding site

How is muscular tension generated?

Through interaction of myosin cross-bridges with thin filaments

What do myomensin and m-protein do?

Vind to titin and help in thick filament assmebly with myosin binding protein C