Figure 2. Mechanisms and consequences of scaffolding adaptors.

Schematized Michaelis-Menten kinetics of substrate degradation illustrating two regimes of substrate concentration: [S] is well below the K_M (A) and well above the K_M (B). If [S] is 100-fold below the K_M, then the degradation rate is ~1% of the maximum velocity (A). Scaffolding adaptors can tether substrates to the protease and increase local concentration by this leashing. As shown in B, a tether length of ~25 residues would result in constraining a single molecule to a volume of ~10⁻²¹ L, which yields an effective concentration of ~1 mM, sufficient to drive degradation at the maximum rate. (C) Substrate recognition by the adaptor must be of at least moderate affinity in order to ensure specificity. However, excessively tight binding of the adaptor restricts delivery of the substrate and inhibits overall degradation.