C505Y

Position 505 sits inside TM6, one of wolframin's eleven transmembrane helices. The AlphaFold model places C505 within 5 Å of PRO504 (2.5 Å), LEU506 (2.5 Å), SER502 (3.6 Å), PRO885 (4.1 Å), VAL503 (4.2 Å), and TYR508 (4.3 Å). The most structurally significant neighbor is PRO885 — that's a residue from TM11 (Atlas card P885L adjacent), positioned 4.1 Å away in the AlphaFold structure. This contact indicates that TM6 and TM11 cross each other in the membrane and pack against one another through the C505/P885 region.

The wild-type cysteine at position 505 is small and hydrophobic-character — its thiol can either remain free or, in oxidizing conditions, participate in disulfide formation. The local environment here is bilayer-embedded so disulfide formation is not the dominant mechanism. The wild-type's small volume fits cleanly into the TM6-TM11 packing interface.

Replacing cysteine with tyrosine here is unusually disruptive for two reasons. First, the volume increase: tyrosine's aromatic ring is roughly four times the side-chain mass of cysteine. The TM6-TM11 interface, which packs tightly through the C505/P885 region, cannot accommodate the larger side chain without significant rearrangement. Second, the introduced hydroxyl is unfavorable in the bilayer hydrophobic core — it would prefer to point toward the membrane-water interface, dragging the local geometry with it.

DynaMut2's |ΔΔG| of 0.81 kcal/mol captures the modest energetic cost of the fold absorbing this rearrangement. But the functional consequence — disrupted TM6-TM11 packing — is more severe than the ΔΔG alone suggests. AlphaMissense's 0.892 score reflects this. Compare with P504L (Atlas card adjacent): proline at position 504 plays a deliberate helix-kinking role, and removing it has its own structural cost. The two variants together — C505Y and P504L — characterize a vulnerable region in TM6 where multiple substitutions produce convergent functional disruption.