Structure of human lactoferrin at 3.2-A resolution

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• Human milk lactoferrin is a key iron-binding protein within the transferrin family.
• Understanding its three-dimensional structure is crucial for elucidating its biological functions.

Purpose of the Study:

• To determine the high-resolution three-dimensional structure of human milk lactoferrin.
• To identify the iron-binding sites and coordination within the lactoferrin molecule.
• To compare the structure of lactoferrin with other related proteins.

Main Methods:

• Crystallographic analysis was employed to determine the molecular structure.
• X-ray diffraction data was collected to a resolution of 3.2 angstroms.
• Electron density maps were analyzed to model the protein structure and identify ligands.

Main Results:

• The three-dimensional structure of human milk lactoferrin was elucidated at 3.2-A resolution.
• The molecule exhibits internal two-fold homology, with two globular lobes (N- and C-terminal halves) each containing an iron-binding site.
• Each lobe consists of two domains with similar supersecondary structure, and the iron-binding site is located at the interface between these domains.
• Iron is coordinated by two tyrosines, one histidine, and one aspartate residue.
• A carbonate or bicarbonate ion was identified near the iron-binding site, interacting with an arginine side chain and a helix N-terminus.

Conclusions:

• The determined structure reveals a conserved folding pattern and iron-binding mechanism within the transferrin family.
• Structural similarities were observed between human milk lactoferrin and other binding proteins, such as the sulfate-binding protein from Salmonella typhimurium.
• These findings provide insights into the molecular basis of iron binding and transport by lactoferrin.