AI Insight
CD163 is a scavenger receptor on macrophages that removes free hemoglobin released during red blood cell lysis, thereby preventing oxidative tissue damage. Using cryogenic electron microscopy, Zhou and Higgins reveal that CD163 binds free hemoglobin tetramers through a full trimeric receptor assembly, a structural requirement distinct from the well-characterized haptoglobin-hemoglobin uptake pathway. The study demonstrates that flexibility at a calcium-mediated base, combined with a hinge between receptor domains 2 and 3, allows the receptor arms to conform around structurally diverse ligands by assembling multiple small binding surfaces into functional cradles.
Why it matters
In conditions such as sickle cell anemia and malaria, the haptoglobin-dependent clearance pathway becomes saturated or depleted, making CD163's direct hemoglobin-binding capacity clinically relevant. Understanding the structural basis of this promiscuous binding could inform the development of therapeutic strategies targeting hemolytic diseases or oxidative stress-related pathology.
by Richard X. Zhou, Matthew K. Higgins
The scavenger receptor CD163 detoxifies free hemoglobin released on erythrocyte lysis to prevent oxidative damage. The best understood route for hemoglobin detoxification involves the formation of haptoglobin-hemoglobin complexes that bind CD163 and are internalized into macrophages, resulting in hemoglobin degradation. However, during conditions such as sickle cell anemia or malaria, haptoglobin is depleted. CD163 can then act as a lower-affinity receptor for free hemoglobin. Previous studies revealed that CD163 forms a multimeric “base,” which presents “arms” that form a binding site for haptoglobin-hemoglobin. In this study, we use cryogenic electron microscopy to reveal how human CD163 binds hemoglobin tetramers in a process that, unlike haptoglobin-hemoglobin uptake, requires a full trimeric CD163 assembly to achieve sufficient binding. We reveal how flexibility at the calcium-mediated base, combined with a hinge between receptor domains 2 and 3, allows the arms to wrap around diverse ligands. This brings together multiple small binding surfaces from different domains to form cradles for different ligands. These adaptations allow the scavenger receptor to be promiscuous, protecting us from oxidative damage caused by hemoglobin release in various pathological conditions.
Source: Structural basis for hemoglobin scavenging by CD163 reveals mechanism of ligand promiscuity