Structure of haptoglobin and the haptoglobin-hemoglobin complex by electron microscopy

Area of Science:

• Biochemistry
• Structural Biology
• Biophysics

Background:

• Haptoglobin (Hp) is a human serum protein that binds free hemoglobin (Hb).
• Hp is a tetrameric glycoprotein composed of two heavy (H) chains and two light (L) chains.
• The Hp-Hb complex plays a crucial role in preventing kidney damage by sequestering free Hb.

Purpose of the Study:

• To elucidate the three-dimensional structure and relative orientation of haptoglobin and hemoglobin in their complex.
• To determine the precise binding site and configuration of Hb on the Hp molecule using advanced imaging techniques.

Main Methods:

• Scanning transmission electron microscopy (STEM) was employed to image negatively stained Hp and Hp-Hb complexes.
• Micrographs were analyzed to measure molecular dimensions, inter-chain distances, and angles.
• Crosslinking of Hb was used to pinpoint specific regions within the complex.

Main Results:

• Haptoglobin exhibits a barbell-like structure with spherical head groups (H chains) connected by a filament (L chains).
• In the complex, Hb alpha beta dimers bind to the H chains at a specific angle (127 degrees), restricting the binding site.
• The interhead group distance increases in the complex, and the orientation of Hb dimers was determined.

Conclusions:

• The study provides a detailed structural model of the haptoglobin-hemoglobin complex.
• The trans configuration of Hb binding to Hp significantly influences the interaction dynamics and binding site accessibility.
• These findings enhance our understanding of Hp's role in hemoglobin scavenging and iron homeostasis.