Protein Function- Protein Interactions Modulated by Chemical Energy Flashcards
Molecular motors
Organisms move. Cells move. Organelles and macromolecules within cells move.
Most of these movements arise from: activity of a class of protein based molecular motors= motor proteins
Fueled by chemical energy, usually derived from ATP
Motor proteins undergo cyclic conformational changes that accumulate into a unified, directional force!
Skeletal muscle
- Muscle fiber: large, single, elongated, multinuclear cell
- Each fiber contains about 1,000 myofibrils.
(muscle fibre (cell) –> fascicle (wrapped by perimysium)–> tendon)
The Major Proteins of Muscle Are Myosin and Actin –> contractile force of muscle: generated by the interaction of these 2 proteins (ação concertada da miosina + actina –> distensão e tensão do músculo)
- -> arranged in filaments that undergo transient interactions and slide past each other to bring about contraction.
- -> make up > 80% of the protein mass of muscle.
Myosin
(tão grande que pode ser observada diretamente por microscopia eletrónica)
(540 kDa) has 6 subunits: two heavy chains (220 kDa) and four light chains (20 kDa)
The heavy chains account for much of the overall structure.
At their carboxyl termini, they are arranged as extended helices, wrapped around each other in a fibrous, left- handed coiled coil similar to that of -keratin
At their amino terminus, each heavy chain has a large globular domain containing a site where ATP is hydrolyzed (–> it is the motor domain that makes muscle contraction possible)
The light chains are associated with the globular domains
[cleavage: trypsin–> light + heavy meremyosin, heavy merem. + papain –> S2+ 2 S1 VER LIVRO ]
Thick filaments
Myofibribs Contain Thick Filaments of Myosin:
• In muscle cells, molecules of myosin aggregate to form structures called thick filaments
–> Contain several hundred myosin molecules arranged with their fibrous “tails” associated to form a long bipolar structure.
• The globular domains project from either end of this structure, in regular stacked arrays.
Actin
Monomeric actin, a 42 kDa polypetide, called
G-actin (globular actin) (tem sítio de ligação para ATP)
associates to form a long polymer called
F-actin (filamentous actin)
–> F-actin is a filamentous assemblage of G-actin monomers that polymerize two by two, giving the appearance of two filaments spiralling about one another in a right-handed fashion (organização pseudo-helicoidal)
Thin filaments
- The thin filament consists of F-actin, along with the proteins troponin and tropomyosin.
- The filamentous parts of thin filaments assemble as successive monomeric actin molecules add to one end.
- On addition, each monomer binds ATP, then hydrolyzes it to ADP, so every actin molecule in the filament is complexed to ADP
- This ATP hydrolysis by actin functions only in the assembly of the filaments; it does not contribute directly to the energy expended in muscle contraction!
F-actin Controlled Growth
Several proteins control the growth of actin, ensuring that the filaments grow only when needed (impedem prolongamento= regulam)
ex: Profilin binds to free actin, blocking part
of the site that binds to neighboring actin molecules in a filament, keeping it from adding to filaments, also inhibiting growt
[veneno… bloqueia mol. de controlo! –> filamentos crescem sem parar]
Myosin+ Actin interaction
Each actin monomer in the thin filament can
bind tightly and specifically to one
myosin head group.
–> Myosin Thick Filaments Slide Along Actin Thin Filaments
Muscle structure
- The organization of thick and thin filaments in a myofibril gives it a striated appearance.
- When muscle contracts, the I bands narrow and the Z disks come closer together
The Z-disc/Z-line is the structure formed between adjacent sarcomeres.
A sarcomere= basic unit
of muscle, composed of sliding protein filaments of actin and myosin.
