Subunits composition and allosteric control in Carcinus aestuarii hemocyanin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• Carcinus aestuarii hemocyanin (Hc) exists in two main aggregation states: 24S (90%) and 16S (10%) at neutral pH and physiological calcium concentrations.
• Dissociation of the 24S form by EDTA or changes in pH reveals distinct 16S and 5S components, including monomeric fractions.

Purpose of the Study:

• To investigate the oxygen-binding properties of different Carcinus aestuarii hemocyanin aggregation states and subunit fractions.
• To elucidate the role of each hemocyanin species in the allosteric oxygen-binding behavior of the native 24S aggregate.

Main Methods:

• Differential dissociation of hemocyanin using EDTA and pH adjustments.
• Purification of hemocyanin subunit fractions.
• Oxygen-binding analysis of native 24S-Hc, 16S forms, and purified subunits.
• Application of the Monod-Wyman-Changeux (MWC) model for data analysis.

Main Results:

• The 24S-hemocyanin exhibits allosteric oxygen binding well-described by a nested MWC model.
• The 16S hemocyanin (half-molecules) follows simple MWC model oxygen binding.
• The two 16S hexamers within the 24S aggregate function as coupled allosteric units in the T-state but appear uncoupled in the R-state.

Conclusions:

• The allosteric properties of Carcinus aestuarii 24S-hemocyanin arise from the cooperative and independent binding behaviors of its constituent 16S subunits.
• Understanding hemocyanin subunit interactions is crucial for deciphering complex oxygen transport mechanisms in invertebrates.