Contractile Proteins and Muscle Diseases

Question 1

Describe the electron micrograph of a single myosin molecule

Answer 1

Two-headed structure with a long, thin tail

Question 2

What are the thick and thin filaments associated with the contractile apparatus in skeletal muscle?

Answer 2

Thin filaments = actin

Thick filaments = myosin

Actin are anchored by Z-discs; 6 thin filaments surround each end of 1 thin filament

Question 3

Describe the bipolar assembly of myosin

Answer 3

Bipolar assembly refers to self-assembly at the tails.

Myosin filaments arranged with tails facing each other (middle = bare zone), tails are coiled coil of 2 alpha helices, light chains and N terminus located near heads at hinge region

Question 4

_______ makes up 35% of total protein in muscle cells

Answer 4

Myosin

Question 5

Myosin is made up of ____ polypeptide chains:

_____ heavy chains + ______ light chains

It has a _______ rod domain and _____ globular heads

Answer 5

6

2; 4

Helical; 2

Question 6

Which region of myosin hydrolyzes ATP

Answer 6

Head region

Question 7

The myosin heads are at _____ degrees rotational symmetry and the surrounding thin filaments are at _____ degrees rotational symmetry

Answer 7

30; 60

Question 8

Treatment of muscle myosin with proteases like _______ or ________ creates stable fragments

Answer 8

Papain; trypsin

[papain cleaves at hinge region, trypsin further down the tail]

Question 9

The myosin tails are arranged in a coiled-coil formation, and with every turn you have _________ residues interacting

Answer 9

Hydrophobic

Question 10

The structure of the S1 fragment (head and neck region) of myosin exhibits a _______ ______ domain

Answer 10

P-loop ATPase

Question 11

The structure of the S1 fragment (head and neck region) of myosin exhibits a P-loop ATPase domain. The P-loop serves as the ______ binding site, while the _______ binding site is closer to the head

Answer 11

Nucleotide; actin

Question 12

The structure of the S1 fragment (head and neck region) of myosin exhibits a P-loop ATPase domain. What 2 types of light chains are associated with the neck region?

Answer 12

Essential light chain

Regulatory light chain

Question 13

Part of the myosin molecule moves dramatically (essential light chain and regulatory light chain in neck region) in response to what?

What is the significance of this?

Answer 13

ATP binding, hydrolysis, and product release

This is what propels myosin along an actin filament!

Question 14

What region of the sarcomere corresponds to actin filaments only?

Answer 14

I band

Question 15

What region of the sarcomere contains both myosin and actin?

Answer 15

A band

Question 16

What protein is responsible for anchoring actin filaments at the ends of each sarcomere?

Answer 16

Z-disc

Question 17

What protein is responsible for anchoring myosin to portions of the z disc?

Answer 17

Titin

Question 18

What protein links the bipolar assembly of myosin together?

Answer 18

Myomesin 1 (aka M protein)

Question 19

Characterize the directionality of actin

Answer 19

Minus end = pointed end

Plus end = barbed end - this is what sticks to the z-disc

Question 20

_______-actin assembles into _______-actin (or filamentous actin), which exists in a ______ ______ arrangement with 2 parallel strands that twist around each other.

Answer 20

G; F; double helical

Question 21

Each G-actin monomer interacts with ____ neighbors. The growing and the final filament exhibits directionality (plus and minus end).

ATP binds ____ G-actin first. The mature filament contains _______ bound to the monomeric G-actin units

Answer 21

Four

Free; ADP

Question 22

Which end of actin attaches to the z-disc?

Answer 22

Plus end (barbed end)

Question 23

The size and position of the actin filament is controlled by several proteins.

_______ caps the minus end (not at Z-disc), preventing further polymerization

Answer 23

Tropomodulin

Question 24

The size and position of the actin filament is controlled by several proteins.

_________ (or beta-actinin, a heterodimer), which associates with alpha-actinin, caps at the plus end

Answer 24

CapZ

Question 25

The size and position of the actin filament is controlled by several proteins. _______ (6,669 residues): "ruler" and template for actin polymerization

Answer 25

Nebulin

Question 26

____________ binds tropomyosin; positions the complex on the filament

Answer 26

Troponin T

Question 27

__________ binds actin; inhibits myosin binding

Answer 27

Troponin I

Question 28

_________ binds Ca and relieves inhibition

Answer 28

Troponin C

Question 29

What protein assists in formation of contractile bundle at Z disc - forming bridges between adjacent thin filaments?

Answer 29

Alpha-actinin

Question 30

Indirect immunofluorescence micrographs of isolated sheets of skeletal muscle Z discs using antibodies to alpha-actinin show that it occurs at the _______ of the Z disc _______ protein is distributed about the Z-disc periphery

Answer 30

Interior Desmin

Question 31

Dystrophin is a 2400 kb gene with 3685 amino acid residues and 79 exons. There are numerous isoforms. What accounts for C-termini vs. N-termini variability?

Answer 31

C-termini variability: Alternative exon splicing N-termini variability: Alternative transcription initiation sites

Question 32

Dystrophin is in the same protein family as what other 2 important proteins?

Answer 32

Alpha-actinin | Spectrin

Question 33

Duchenne Muscular Dystrophy and Becker Muscular Dystrophy result from what genetic event?

Answer 33

Exon deletions (may also be exon duplications but that is less common)

Question 34

What is the difference between DMD and BMD?

Answer 34

Duchenne muscular dystrophy - no detectable dystrophin Becker muscular dystrophy - dystrophins of various sizes

Question 35

What is the inheritance pattern for DMD?

Answer 35

X-linked recessive

Question 36

What is the role of dystrophin in the musculoskeletal system?

Answer 36

Dystrophin is a 427 kDa rod-like protein that connects the cytoskeleton to the basal lamina and stabilizes the membranes and participates in calcium handling It is the largest human gene known

Question 37

What mediates the conversion of glycogen to glucose in muscle cells?

Answer 37

Glycogen phosphorylase

Question 38

What mediates the conversion of glucose to pyruvate in muscle cells?

Answer 38

AMP

Question 39

Activation of muscle glycogen phosphorylase occurs by ______ and by phosphorylation. Muscle glycogen phosphorylase is composed of _____ identical subunits. AMP binds to the ________ site, a site separate from the active catalytic site. _________ ________ can phosphorylate a specific serine residue on each subunit. Either phosphorylation or binding of AMP causes a change in the active site that increases the activity of the enzyme

Answer 39

AMP; two Allosteric Phosphorylase kinase

Question 40

In Tarui disease (glycogen storage disease VII), the muscle isoform of the rate limiting enzyme of glycolysis is defective. This enzyme is subject to allosteric control; it is activated by some and inhibited by other modulatos. To make a definitive diagnosis of Tarui disease, the activity of this enzyme in muscle biopsies needs to be measured. Because the amount of tissue in a biopsy is very small, the assay needs to be as sensitive as possible (i.e., all activating), but none of the inhibiting modulators should be present. Which of the following inhibits the enzyme and should be left out of the assay mixture? A. ADP B. AMP C. Citrate D. Fructose-6-Phosphate

Answer 40

C. Citrate

Question 41

What enzyme is deficient in Tarui disease? What tissue does this disease affect?

Answer 41

PFK Affects muscle tissue

Question 42

What are the clinical features of Tarui disease, and what disease are its symptoms similar to?

Answer 42

Clinical features: exercise intolerance, muscle cramping, exertional myopathy, compensated hemolysis, and myoglobinuria Symptoms can be similar to McArdle's Glycogen Storage Disease, but more severe

Question 43

A 25 y/o african american male notices increased fatigue and decreased performance when playing outfield for his office softball team. After a game in hot weather with many fielding chances, he has severe muscle pains and cramps that keep him awake and he notices that his urine is dark that night. His physician suspects McArdle disease (type V glycogen storage disease) and has him run on a treadmill breathing air with reduced oxygen. The man develops severe cramps after this ischemic exercise, further supporting the presumptive diagnosis. A diagnostic muscle biopsy is likely to show which of the following? A. Increased normal glycogen with a deficient hexokinase B. Increased normal glycogen with a deficient glycogen synthase C. Increased normal glycogen with a deficient phosphorylase D. Decreased muscle glycogen with decreased glycogen synthase E. Decreased muscle glycogen with decreased debranching enzyme

Answer 43

C. Increased normal glycogen with deficient phosphorylase

Question 44

What are the 3 possible fates of glucose 6-phosphate?

Answer 44

1. Used as fuel for anaerobic or aerobic metabolism (fed into glycolysis) 2. Converted into free glucose in the liver and then released into blood (using glucose 6 phosphatase) 3. Processed by pentose phosphate pathway to generate NADPH or ribose

Question 45

__________ from the adrenal medulla enhances glycogen breakdown in muscle and liver to provide fuel for muscle contraction

Answer 45

Epinephrine