The primary structure of donkey (Equus asinus) lysozyme contains the Ca(II) binding site of alpha-lactalbumin.

The research involved studying the primary structure of donkey lysozyme and it discovered that it contains a full Calcium (II) binding site similar to alpha-lactalbumins and has half the similarity to the human protein.

Research Methodology

• The research began with pulsed liquid-phase sequencing which determines the order of amino acids in a protein captured from tryptic and chymotryptic peptides. These peptides were isolated by using Reverse Phase-High Performance Liquid Chromatography (RP-HPLC), a commonly used protein purification technique.
• Cysteine residues – one of the 20 amino acids forming a protein – were identified by labeling them with DABIA-reagent, a chemical used to specifically tag cysteine residues.

Findings

• The donkey lysozyme was identified as a c-type lysozyme consisting of 129 amino acids.
• There was a 50% homology (similarity) to the human protein, implying that half the sequence of amino acids in donkey lysozyme is the same as in the human version of the protein.
• The research identified a full Calcium(II) binding site, which is a binding location for calcium ions within the protein structure. This site was previously suggested for alpha-lactalbumins, proteins usually found in milk.

Comparative Analysis

• A comparison was made with horse lysozyme, another c-type lysozyme. It was noted that while the horse lysozyme contains asparagine in position 61, all known c-type lysozymes, including the donkey lysozyme, contain serine (Ser) in the same position. This caused a conflict with the existing three-dimensional structure of lysozyme and stands as an assertion for further research.