You get into an argument with your classmate about whether ATP hydrolysis is required for actin polymerization. You argue that ATP hydrolysis is not required, because ADP can, under certain conditions, substitute for ATP, enabling the formation of ADP-actin filaments. Your friend counter-argues that ADP-actin filaments are less stable than ATP-actin filaments, so therefore it is the free energy of ATP hydrolysis that drives actin assembly. In other words, your friend believes that the hydrolysis of ATP-actin to ADP-Pi-actin is a necessary step in polymer formation.

How do you respond?

Although actin hydrolyzes ATP, everything seems to indicate that this does not intervene in the assembly, since, on the one hand, hydrolysis occurs largely inside the filament, and on the other hand, ADP can also polymerize.

Explanation:

This raises the debate of understanding what is the thermodynamically unfavorable process that requires such a huge energy expenditure. The so-called "actin cycle", which links hydrolysis to polymerization, consists of the addition of Actin G-ATP monomers preferably at the bearded end, creating a flow of monomers towards the arrowhead end where known as "braiding" where the monomers would be in the form of Actin F-ADP and would be released, subsequently exchanging this ADP for ATP and thereby closing the cycle.

Shortly after addition, ATP hydrolysis occurs relatively quickly. There are two hypotheses on how it occurs, stochastic, in which hydrolysis would occur randomly influenced in some way by neighboring molecules, and vector, in which it would only occur at the limit with other molecules that have already hydrolyzed their ATP. In any case, the resulting Pi is not released, but remains for a time non-covalently bound to actin ADP, with which there would be three species of actin in a filament: ATP-Actin, ADP + Pi-Actin and ADP-Actin. The content of a filament in each of these species depends on its length and condition: at the beginning of elongation, the filament has an approximately equivalent composition of monomers with ATP and ADP + Pi and a small amount near the (-) end of Actin ADP. As steady state is reached, the situation is reversed, with most of the filament being ADP and the (+) end practically only ADP + Pi, with ATP reduced to the extreme.

If we compare the pure actin-ADP filaments with those that incorporate ATP, in the first the constant constants are similar at both ends, while in the other two nucleotides the Cc is different:

At the (+) end it is Cc + = 0.1 μM, while at the (-) end it is Cc - = 0.8 μM, which gives the following situations:

-For actin G-ATP concentrations lower than Cc +, filament elongation does not occur.

-For actin G-ATP concentrations less than Cc - but greater than Cc +, elongation occurs at the (+) end.

For actin G-ATP concentrations greater than Cc - the microfilament grows at both ends.