T

3) transport of substrate causes dephosphorylation of carrier protein and closure of transport pore

Figure 3.7 The role of ATP in active transport — P-type transporters which have to bephosphorylated in order to permit transport across the cell membrane.

H3PO4

h2o inside cell

membrane outside cell H2O _ -

membrane ATPase

substrates the sodium pump t

sodium cotransport

substrates Na+

sodium counter-transport exports waste

Figure 3.8 The role of ATP in active transport — generation of a proton gradient linked to the sodium pump and sodium-dependent transporters.

the membrane, and takes an H+ ion from inside the cell and an OH- ion from the extracellular fluid, so creating a pH gradient across the membrane.

The protons in the extracellular fluid then enter the cell on a transmembrane carrier protein, and react with the hydroxyl ions within the cell, so discharging the pH gradient. The carrier protein that transports the protons across the cell membrane does so only in exchange for sodium ions. The sodium ions in turn re-enter the cell either:

  • Together with substrates such as glucose and amino acids, thus providing a mechanism for net accumulation of these substrates, driven by the sodium gradient, which in turn has been created by the proton gradient produced by the hydrolysis of ATP This is a co-transport mechanism as the sodium ions and substrates travel in the same direction across the cell membrane;
  • In exchange for compounds being exported or excreted from the cell. This is a counter-transport mechanism, since the sodium ions and the compounds being transported move in opposite directions across the membrane.

3.2.3 The role of ATP in muscle contraction

The important proteins of muscle are actin and myosin. As shown in Figure 3.9, myosin is a filamentous protein, consisting of several subunits, and with ATPase activity myosin the arrangement of myosin in myofibrils

ATPase in myosin light chains actin troPo m yosin fibre troponin

(calcium-binding regulatory protein)

Figure 3.9 The contractile proteins of muscle.

in the head region. In myofibrils, myosin molecules are arranged in clusters with the tail regions overlapping. Actin is a smaller, globular protein, and actin molecules are arranged around a fibrous protein, tropomyosin, so creating a chain of actin molecules, interspersed with molecules of a calcium-binding regulatory protein, troponin.

In resting muscle, each myosin head unit binds ADP, and is bound to an actin molecule, as shown in Figure 3.10. The binding of ATP to myosin displaces the bound ADP and causes a conformation change in the molecule, so that, while it remains associated with the actin molecule, it is no longer tightly bound. Hydrolysis of the bound ATP to ADP and phosphate causes a further conformational change in the myosin molecule, this time affecting the tail region, so that the head region becomes associated with an actin molecule further along. This is the power stroke which causes the actin and myosin filaments to slide over one another, contracting the muscle filament. When the phosphate is released, the head region of myosin undergoes the reverse conformational change, so that it now becomes tightly bound to the new actin molecule, and is ready to undergo a further cycle of ATP binding, hydrolysis and movement.

myosin

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