Biochemistry: Muscles Flashcards
What are the functions of muscles?
- body moving
- respiration
- digestion
- vascular tone and blood circulation
- excretion
- kinetics (mechanical work)
What two groups are vertebrate muscles classified into?
- skeletal muscles
- smooth muscles
What three compounds play a major role in muscle contraction?
- myofibril proteins
- Ca2+
- ATP
What is each muscle fiber covered by?
membrane (sarcolemma)
What does the sarcolemma consist of?
50-70% proteins:
- ion channels
- enzymes
- pumps
- structural proteins
- etc.
What follows the sarcolemma?
the sarcoplasm
What is the sarcoplasm? What is it composed of?
an intracellular fluid
- sarcoplasmic reticulum
- transverse pipe system
- myofibrils
- energy sources (lpids, glycogen molecules)
- enzymes
What are myofibrils?
complex of contractile proteins
What are skeletal muscles composed of?
72-80% water
20-28% dry matter
What do intracellular muscle proteins divide into?
sarcoplasma proteins (water soluble)
fibrillar myofibril proteins (insoluble in water)
What is the function of sarcoplasma proteins? Examples.
MYOGLOBIN:
accumulate oxygen in muscles
(the colour of the muscles depends on it)
CALCEZVESTRINE:
stimulate the onset of muscle contraction
State an example of a sarcoplasma protein.
myoglobin
calcezvestrine
What are myofibrils?
complex of contractile proteins
What are the two types of myofillaments that the myofibrils consist of? What do they consist of?
1) thick fillaments: 200-400 molecules of myosin
2) thin fillaments: complex of actin proteins, tropomyosin and troponin
What percentage of all proteins involved in muscle contraction does myosin, actin and troponin account for?
90%
What is myosin? What is it composed of?
- a molecule
- fibrillar “tail“: two identical twisted alpha helices (form a supercoil)
- globular “head”
What is the function of myosin?
1) myosin mollecules bind to fillaments under physiological conditions.
400 myosin molecules make up a myosin fillament.
2) myosin has ATPase activity.
- hydrolyses ATP (used for muscle contraction)
3) myosin is a mechanoenzyme
- catalyses the conversion of energy from chemical bonds into mechanical energy
What is actin?
- monomeric, globular protein
- composed of a single polypeptide chain of 374 amino acids
- not specific (also present in other eukaryotic cells)
What does the actin fillament consist of?
- actin
- tropomyosin (Tm)
- troponin’s (3)
What does the actin core consist of?
two twisted F-actin chains (fibrillar actin)
(each consisting of 400 polymerised G-actin molecules)
What is attached to the F-actin fillament? By what joints? Where located?
2 chains of tropomyosin molecules
- by flexible joints
- located in a depression formed by two actin chains
What is tropomyosin bound to?
3 troponin proteins:
- Troponin C (Ca2+ binding)
- Troponin I (inhibitory)
- Troponin T (tropomyosin binding)
What is the name of the compound formed when the heads of the myosin molecule bind to the actin threads?
actomyosin
What are muscle nitrogenous compounds? Explain their molecular weight. What is their function? Examples?
- organic componds that contain nitrogen,
- low molecular weight
- regenerate ATP, reduce muscle fatigue, increase the amplitude of muscle contraction,
- eg. amino acids, nucleotides, creatine
What are muscle non-nitrogenous compounds? What is their function? Examples?
- organic componds that do not contain nitrogen,
- reserve source of energy (triglycerides), muscle contractions (ions)
- eg. glucogen, milk, pyruvate, lipids, inorganic salts
How does the electrical impulse reach the muscles?
through the junction of nerve and muscle in the sarcolemma
What is the usual potential difference of the sarcolemma? What happens when there is an electrical impulse?
- excess of politive charge on the outside
- inside: -60mV
DEPOLARIZATION:
- sudden increase in membrane conductivity to K+, Na+ and Ca2+ ions.
What ions are displaced into the sarcoplasm due to depolarization? What does this rapid displacement cause?
Ca2+ ions
- causes ATP to be hydrolysed (allowing for muscle contraction)
What happens to myosin when tropomyosin changes its position?
the myosin head (containing ADP and Pi) rotates at a large angle to join the F-actin. to form an angle of 90 degrees.
What happens during the ‘power stroke’? How does it occur?
fusion of myosin, ATP, and actin releases Pi.
- myosin changes angle from 90 degrees to 45 degrees, by pulling actin towards it
Why does the myosin head position change to 45 degrees?
it is the conformation which requires the least energy
What happens in stage 3? How is the myosin head detatched from the actin?
ATP binds to the myosin-F-actin complex
What happens in step 4? Once the ATP attatches to myoactin complex?
the newly bound ATP is hydrolysed by myosin and a new interaction occurs without releasing ADP and Pi.
The cycle repeats.
What is the function of ATP in the sliding fillament theory?
ATP seperates the myosin head from the thin fillament (F-actin) and is the driving force behind muscle contraction.
What is the main factor regulating muscle contraction? What concentration of Ca2+ is there when?
Ca2+
- highest muscle contraction: 10^-6/5
- no contraction: 10^-7
How is smooth muscle different from skeletal muscle?
- NO TROPONIN COMPLEXES.
- chains of MYOSIN differ (phosphorylated or dephosphorylated)
- ACTIN > MYOSIN (reversed ratio in transverse muscles)
What are the three types of smooth muscles?
1) individual unit
2) multiple muscle units
2) mixed forms
Explain the individual muscle unit. State examples.
- smooth cells connect through slit joints
- react together as a seperate functional unit
eg. digective tract, bladder, ureter, uterus
Explain multiple units of smooth muscle. Examples.
- connected by the autonomic nervous system
- actions are independent of other cells or groups of cells in the same muscle
- eg. iris, dendate muscles
Explain mixed forms of smooth muscle. Examples.
- both types of properties
- activity generated by the nervous system and smooth muscles themselves
- eg. vascular
