The Selective Door

D-cysteine is the mirror-image form of the amino acid cysteine. Normal biology uses L-cysteine. The D-form doesn't belong — it isn't incorporated into proteins, isn't part of standard metabolism. It's the wrong-handed version of a common molecule.

Certain cancer cells express a transporter on their surface that healthy cells lack. D-cysteine fits this transporter precisely because of its "wrong" chirality. Once inside, it blocks NFS1, a mitochondrial enzyme critical for iron-sulfur cluster production, eventually killing the cell.

The through-claim: the selectivity is in the access, not the action. D-cysteine doesn't discriminate between healthy and cancerous cells at the level of its biochemical effect. NFS1 inhibition would damage any cell. The selectivity comes entirely from the membrane — which cells let it in. When researchers artificially expressed the transporter on healthy cells, those cells died too. The molecule is indiscriminate. The door is selective.

This inverts the standard drug design paradigm, which optimizes the key — finding molecules that bind specifically to disease-associated targets. Here the key is generic (it kills any cell it enters). The lock is specific (only cancer cells have the door). Target the lock, not the key.

The wrong-handedness is the enabling feature. L-cysteine, the "correct" enantiomer, would enter all cells through standard amino acid transporters — no selectivity. D-cysteine is ignored by normal transporters and recognized only by the cancer-specific one. The chirality that makes it biologically useless in general is what makes it therapeutically precise.