The primary structure of monomeric beta-lactoglobulin I from horse colostrum (Equus caballus, Perissodactyla).

This research details the identification and sequencing of a particular protein, beta-lactoglobulin I, found in horse colostrum and milk. The protein was detected and then analyzed using a range of methods, and its difference from a similar protein found in bovines was highlighted.

Protein Detection and Analysis

• The mammals’ milk proteins, specifically from horses, were probed for the existence of beta-lactoglobulin-like proteins. These proteins, contrary to others that have been previously analyzed, were found to be monomeric, which means they are composed of single units, unlike polymers that consist of multiple units.
• The structure of these proteins was also found to be genetically dissimilar, implying possible variations in the genetic code that creates these proteins.

Sequencing of Beta-Lactoglobulin I

• The investigators report the primary structure or sequence of the monomeric beta-lactoglobulin protein obtained from horse colostrum, using an automatic liquid-phase sequenator. This involves determining the order of the amino acids that make up the protein.
• Tryptic digestion, cyanogen bromide cleavage, limited tryptic digestion, and hydrolysis with chymotrypsin were used to achieve the necessary overlapping peptides, therefore revealing the sequence of peptides
• The sequencing revealed that the horse beta-lactoglobulin I consists of 162 amino acids that include four cysteine residues, six methionine residues, and one tryptophan residue.

Comparison with Bovine Beta-Lactoglobulin A

• A homologous comparison to bovine beta-lactoglobulin A showed significant differences, with a variance of 72 amino acids between the two, constituting 44% difference. This shows that although the proteins may perform similar functions, their physical structures are notably different.
• Despite obvious differences, some similarities were also identified. For instance, thirteen of the amino acid exchanges could be explained as two-point mutations.
• A significant difference, however, was found in beta-lactoglobulin. The free thiol group found in bovine beta-lactoglobulin was lacking in the horse version. Instead, a tyrosine substitution was found at position 121 previously occupied by cysteine in the bovine version.