Answer
Around 4 or 5 hydrogen bonds would be needed
Work Step by Step
Interacting proteins that are present in cells in lower concentrations need to bind to each other with high affinities so that a significant fraction of the molecules are bound at equilibrium. In this particular case, lowering the concentration by three orders of magnitude (from uM to nM) requires a change in the equilibrium constant by three orders of magnitude to maintain the AB protein complex (corresponding to -4.3 kcal of free energy; see Table 3-1). This corresponds to about four or five extra hydrogen bonds.
