Contractile Proteins and Muscle Diseases Flashcards
1
Q
Describe the electron micrograph of a single myosin molecule
A
Two-headed structure with a long, thin tail
2
Q
What are the thick and thin filaments associated with the contractile apparatus in skeletal muscle?
A
Thin filaments = actin
Thick filaments = myosin
Actin are anchored by Z-discs; 6 thin filaments surround each end of 1 thin filament
3
Q
Describe the bipolar assembly of myosin
A
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
4
Q
_______ makes up 35% of total protein in muscle cells
A
Myosin
5
Q
Myosin is made up of ____ polypeptide chains:
_____ heavy chains + ______ light chains
It has a _______ rod domain and _____ globular heads
A
6
2; 4
Helical; 2
6
Q
Which region of myosin hydrolyzes ATP
A
Head region
7
Q
The myosin heads are at _____ degrees rotational symmetry and the surrounding thin filaments are at _____ degrees rotational symmetry
A
30; 60
8
Q
Treatment of muscle myosin with proteases like _______ or ________ creates stable fragments
A
Papain; trypsin
[papain cleaves at hinge region, trypsin further down the tail]
9
Q
The myosin tails are arranged in a coiled-coil formation, and with every turn you have _________ residues interacting
A
Hydrophobic
10
Q
The structure of the S1 fragment (head and neck region) of myosin exhibits a _______ ______ domain
A
P-loop ATPase
11
Q
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
A
Nucleotide; actin
12
Q
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?
A
Essential light chain
Regulatory light chain
13
Q
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?
A
ATP binding, hydrolysis, and product release
This is what propels myosin along an actin filament!
14
Q
What region of the sarcomere corresponds to actin filaments only?
A
I band
15
Q
What region of the sarcomere contains both myosin and actin?
A
A band
16
Q
What protein is responsible for anchoring actin filaments at the ends of each sarcomere?
A
Z-disc
17
Q
What protein is responsible for anchoring myosin to portions of the z disc?
A
Titin
18
Q
What protein links the bipolar assembly of myosin together?
A
Myomesin 1 (aka M protein)
19
Q
Characterize the directionality of actin
A
Minus end = pointed end
Plus end = barbed end - this is what sticks to the z-disc
20
Q
_______-actin assembles into _______-actin (or filamentous actin), which exists in a ______ ______ arrangement with 2 parallel strands that twist around each other.
A
G; F; double helical
21
Q
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
A
Four
Free; ADP
22
Q
Which end of actin attaches to the z-disc?
A
Plus end (barbed end)
23
Q
The size and position of the actin filament is controlled by several proteins.
_______ caps the minus end (not at Z-disc), preventing further polymerization
A
Tropomodulin
24
Q
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
A
CapZ
25
The size and position of the actin filament is controlled by several proteins.
_______ (6,669 residues): "ruler" and template for actin polymerization
Nebulin
26
____________ binds tropomyosin; positions the complex on the filament
Troponin T
27
__________ binds actin; inhibits myosin binding
Troponin I
28
_________ binds Ca and relieves inhibition
Troponin C
29
What protein assists in formation of contractile bundle at Z disc - forming bridges between adjacent thin filaments?
Alpha-actinin
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
Interior
Desmin
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?
C-termini variability: Alternative exon splicing
N-termini variability: Alternative transcription initiation sites
32
Dystrophin is in the same protein family as what other 2 important proteins?
Alpha-actinin
| Spectrin
33
Duchenne Muscular Dystrophy and Becker Muscular Dystrophy result from what genetic event?
Exon deletions (may also be exon duplications but that is less common)
34
What is the difference between DMD and BMD?
Duchenne muscular dystrophy - no detectable dystrophin
Becker muscular dystrophy - dystrophins of various sizes
35
What is the inheritance pattern for DMD?
X-linked recessive
36
What is the role of dystrophin in the musculoskeletal system?
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
37
What mediates the conversion of glycogen to glucose in muscle cells?
Glycogen phosphorylase
38
What mediates the conversion of glucose to pyruvate in muscle cells?
AMP
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
AMP; two
Allosteric
Phosphorylase kinase
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
C. Citrate
41
What enzyme is deficient in Tarui disease? What tissue does this disease affect?
PFK
Affects muscle tissue
42
What are the clinical features of Tarui disease, and what disease are its symptoms similar to?
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
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
C. Increased normal glycogen with deficient phosphorylase
44
What are the 3 possible fates of glucose 6-phosphate?
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
45
__________ from the adrenal medulla enhances glycogen breakdown in muscle and liver to provide fuel for muscle contraction
Epinephrine
46
Glycogen degradation is stimulated by hormone binding to _________, which initiates a signal transduction pathway that results in the phosphorylation and activation of _________ __________
7-transmembrane GPCR
Glycogen phosphorylase
47
Activation of PKA leads to phosphorylation of phosphorylase kinase. Is the phosphorylated version of phosphorylase kinase active or inactive?
Active
NOTE that calcium is also required to make phosphorylase kinase fully active
[protein phosphatase 1 or 2 removes this phosphate, making the phosphorylase kinase inactive]
48
T/F: The regulatory mechanisms of phosphorylase kinase vary somewhat in liver vs. muscle
True
49
In muscle cells, phosphorylation of the alpha and beta subunits by PKA is the result of cAMP-mediated signaling cascade initiated by the binding of ________ to beta-adrenergic receptors on the cell surface.
Additionally, the release of ______ from the sarcoplasmic reticulum during muscle contraction _________ the inhibitory delta subunit and activates PhK fully
Epinephrine
Ca2+
Inactivates
50
In liver cells, both ________ and epinephrine can trigger the cAMP-PKA cascade, while epinephrine also binds the ____________ receptor to trigger a phosphoinositide cascade, resulting in the release of Ca++ from the ER
Glucagon
Alpha-adrenergic
51
When the cell needs to stop glycogen breakdown, PhK is dephosphorylated by the protein phosphatases 1 and 2, returning the ______ and ______ subunits to their initial inhibitory configuration
Alpha; beta
52
Phosphorylase kinase is partly activated by calcium binding to the ____ subunit. Activation is maximal when the ____ and ____ subunits are phosphorylated in reponse to hormonal signals.
When active, the enzyme converts phosphorylase ___ into phosphorylase ___
Delta
Alpha; beta
B --> A
53
What are the positive and negative regulators on the rate limiting enzyme of glycolysis?
RL enzyme = PFK-1
Positive regulators: AMP, Fructose 2,6-BP
Negative regulators: ATP, Citrate
54
In glycolysis, most enzymatic reactions are reversible and can also be used for gluconeogenesis. However, there are 3 glycolytic reactions that are irreversible, and different enzymes are required to bypass them in gluconeogenesis. Which of the following enzymes catalyzes such an irreversible step?
```
A. Phosphoglucose isomerase
B. PFK-1
C. Aldolase A
D. Glyceraldehyde-3-phosphate dehydrogenase
E. Phosphoglycerate kinase
```
B. PFK-1
55
A 7 y/o girl is brought to the physician because of weakness and muscle cramping after playing in the playground with her playmates. Physical exam shows no abnormalities and blood tests reveal no hypoglycemia. Skeletal muscle biopsy shows increased levels of glycogen stores of normal structure. This patient most likely has a deficiency of which of the following enzymes?
```
A. Branching enzyme
B. Debranching enzyme
C. Glycogen phosphorylase
D. Glycogen synthase
E. UDP-glucose pyrophosphorylase
```
C. Glycogen phosphorylase
[McArdle disease, this is the RL enzyme of glycogenolysis in skeletal muscle]
56
A muscle phosphorylase deficiency associated with McArdle's disease is also known as a __________ deficiency
Myophosphorylase
57
Mutations in the muscle gene for glycogen phosphorylase cause ________ syndrome, while mutations in liver gene for glycogen phosphorylase cause _______ disease
McArdle; Hers
58
In comparing muscle vs. liver glycogen phosphorylase:
Both forms are activated by phosphorylation by _________ _______.
Both forms are inhibited allosterically by ________ and ________
Phosphorylase kinase
ATP; glucose 6-phosphate
59
In comparing muscle vs. liver glycogen phosphorylase:
Both forms are activated by phosphorylation by PhK. Both forms are allosterically inhibited by ATP and glucose-6P.
The muscle form is allosterically activated by what 3 things?
AMP (low cellular energy)
Calcium-calmodulin complex
G-actin
60
The liver form of glycogen phosphorylase is inactivated by ______ _____, but is not affected by _____
Free glucose; AMP
61
Muscle glycogen phosphorylase is allosterically activated by AMP, while the liver form is not affected by AMP.
Brain astrocytes also utilize glycogen phosphorylase. Is the brain-specific glycogen phosphorylase responsive to AMP?
NO!
The brain contains small amounts of glycogen
62
Glycogen phosphorylase b and phosphorylase a exist as equilibria between an active ____ state and a less active ____ state.
Phosphorylase b is usually inactive because the equilibrium favors the _____ state.
Phosphorylase a is usually active because the equilibrium favors the ____ state.
R; T
T
R
63
What conditions favor the transition of glycogen phosphorylase to the R state in muscle?
What stabilizes the T state?
A low energy charge (high AMP concentration)
T state is stabilized by ATP and glucose 6P
64
What conditions favor the T state in liver glycogen phosphorylase?
The binding of glucose to phosphorylase A shifts the equilibrium to the T state and inactivates the enzyme
Thus, glycogen is not mobilized when glucose is already abundant
65
The muscle-specific glycogen phosphorylase are very sensitive to activation by calcium. What are some ways that calcium is released so that this mechanism can take place?
Ca++ released from SR during muscle contraction (NMJ depolarization)
Ca++ release is also stimulated by epinephrine through inositol triphosphate (IP3) signaling
66
_________ is a muscle relaxant drug that binds to ryanodine receptors and prevents the mobilization of calcium to prevent the activation of phosphorylase a.
Thus, it can be used to treat _______ _______ induced by general anesthesia in susceptible patients
Dantroline
Malignant hypothermia
67
__________ _________ are huge ion channels responsible for the release of calcium from the SR. They form homo-tetramers with a mushroom like shape and consist of a large cytoplasmic head and a transmembrane stalk
Ryanodine receptors
68
What conditions increase susceptibility to malignant hyperthermia?
Mutations in ryanodine receptors
Mutations in L-type Voltage Gated Calcium channels
[reabsorption of mobilized calcium requires large amounts of ATP which leads to hyperthermia in muscle]
69
Thin filaments from adjacent sarcomeres are crosslinked by _________ at the Z-disc. 6 giant ____ proteins lie along the entire length of the thick filament and beyond, spanning the center of the sarcomere to Z-disc, where they interact with the protein in the first blank
Alpha-actinin; titin
70
A 14 y/o boy with a hx of muscle disease comes to the physician for a follow-up examination. He is confined to a wheelchair and has had progressive muscle weakness, especially of his legs. He says that he did not have any trouble moving his arms, but now they appear to be affected. Serum studies demonstrate elevated levels of a creatine kinase; dipstick urinalysis is negative for blood. Which of the following would a muscle biopsy specimen most likely show?
A. Glycogen accumulation within the myocytes
B. Irregular muscle fibers with abnormal mitochondria
C. Large amount of lipid droplets within the myocytes
D. Necrosis and phagocytosis of muscle fibers
E. Panfascicular atrophy
D. Necrosis and phagocytosis of muscle fibers
[DMD = wheelchair bound by age 12, death by cardiac or respiratory failure by 20; elevated serum creatine kinase, first muscles affected are proximal legs, upper extremities later; affected muscles show nerosis and phagocytosis of muscle fibers on biopsy]
71
True/False:
```
Alpha-DG
Beta-DG
SGC/SPN
Alpha-Db 2
SYN
```
Above are constituents of the core dystrophin-glycoprotein complex, which co-purify as a highly stable complex from skeletal muscle and which show greatly decreased abundance in dystrophin-deficient muscle.
True
[note that there are also proteins like talin, vinculin, dysbinding, syncoilin, etc. that are present at increased levels when dystrophin is absent]
72
Duchenne's muscular dystrophy is associated with loss of _______
Dystrophin
73
What are the 2 primary proteins involved in lateral and longitudinal force transfer?
Dystrophin
| Desmin
74
The third domain of DMD contains _____-rich domains
Cysteine
75
Dystrophin contains an amino-terminal actin binding domain consisting of a tandem CH domain, a spectrin-like triple helical repeat domain with 4 putative hinge modules, a _______ rich domain critical for binding beta-dystroglycan, and a carboxy-terminal domain important for binding _________ and alpha-dystrobrevin
Cysteine
| Syntrophins
76
Describe the arrangement and function of the dystrophin protein
Dystrophin is localized to the cytoplasmic face of the muscle cell plasma membrane (sarcolemma), and particularly within a cytoskeletal lattice termed costameres.
Through an extensive network of interacting proteins costameres physically couple the sarcolemma with the Z disk of force-generating myofibrils
77
Describe the use of Troponin as a cardiac enzyme indicator following a myocardial infarction
Troponin is a protein complex that plays a role in muscle contraction and relaxation triggered by elevated or depressed levels of intracellular calcium respectively.
It is made up of 3 subunits: T, C, and I. Upon calcium binding, the protein changes conformation which transmits to tropomyosin so that myosin can bind actin filaments and contraction will occur. Cardiac troponin I (cTn-I) is found in cardiac muscle and its serum levels increase after an MI
78
What disease is characterized by a deficiency in either the catalytic subunit of glucose 6 phosphatase or ER glucose 6 phsophate translocas?
Von Gierke's disease
79
What disease is characterized by a mutation in the glycogen acid alpha-glucosidase which controls the formation of the enzyme acid maltase, thus leading to skeletal muscle and cardiac abnormalities as well as glycogen accumulation in all orgens?
Pompe (type II) glycogen storage disease
80
For Pompe glycogen storage disease, cardiac involvement is the most prominent feature of _______ onset cases and skeletal muscle weakness is conspicuous in ______-onset cases
Infantile; adult
81
What disease is characterized by a buildup of glycogen in liver and muscle and infants are born with hypoglycemia, hyperlipidemia, and elevated liver enzymes followed by hepatomegaly, chronic liver disease, and liver failure, as well as short stature and hypotonia?
Cori's disease (type III GSD)
82
What disease is characterized by a deficiency in glycogen branching enzyme affecting primarily the liver?
Andersen's disease (type IV GSD)
83
What disease is characterized by a deficiency in liver glycogen phosphorylase leading to failure to thrive, hepatomegaly, and mild hypoglycemia?
Hers disease (type VI GSD)
84
What disease is characterized by a deficiency in the muscle glycogen phosphorylase , limiting ATP generation by glycogenolysis and resulting in glycogen accumulation?
McArdle disease (Type V GSD)
85
What disease is considered indistinguishable from McArdle disease except for the absence of the "second wind" phenomenon, and useful labs are increased bilirubin and reticulocyte count, reflecting a compensated hemolysis?
PFK deficiency aka Tarui- GSD type VII
86
Pseudohypertrophy of calf muscles is a classic sign of what myosinopathy?
Duchenne muscular dystrophy
87
What form of muscular dystrophy occurs in late childhood adolescence and manifests with a nearly normal lifespan?
Becker muscular dystrophy
88
What type of inheritance pattern would you find with a mitochondrial myopathy?
Maternal inheritance
Mutations often occur in both nuclear and mitochondrial genes
89
_______ = sustained involuntary contraction of a group of muscles; can be elicited by percussion on thenar eminence
Myotonia
