Partial amino-acid sequence and cysteine reactivities of cytosolic aspartate aminotransferase from horse heart.

The research describes the to partial amino-acid sequence and cysteine reactivity of the enzyme – Cytosolic aspartate aminotransferase found in horse heart, comparing its similarities with the pig heart isoenzyme and highlighting the unique trait of the horse heart enzyme being acetylated at the N-terminus.

Research Context and Methodology

• The research focuses on Cytosolic aspartate aminotransferase (EC 2.6.1.1) from horse heart, an enzyme involved in critical metabolic processes.
• The study determined that this enzyme has five cysteine residues. Researchers examined the reactivity of these residues by titration with 5,5′-dithiobis(2-nitrobenzoid acid).
• The reactivity was measured in the native enzyme state without any impairment of its catalytic activity. The rate of this modification was found not affected by the presence of substrates.
• Further, the research led to the loss of catalytic activity when the enzyme was reacted with N-ethylmaleimide, with the rate of inactivation being increased by the presence of substrates.

Key Findings

• Through sequence analysis, peptides containing 361 amino-acid residues, approximately 88% of the protein’s total number, were isolated and aligned to compare with the known sequence of the isotopic isoenzyme from pig heart.
• In the areas of comparison, 342 of the residues were found to be identical. This finding suggests a high degree of structural similarity, around 95% structural identity, between the cytosolic isoenzymes from horse and pig heart, given the regions are representative of the whole protein sequence.
• However, one critical difference observed was that, uniquely among the mammalian cytosolic aspartate aminotransferases explored, the enzyme from horse heart is acetylated at the N-terminus. This aspect showcases a distinct feature that this particular enzyme variant possesses, potentially hinting at different functional or regulatory properties unique to the horse heart enzyme.

Study Implications

• This study is foundational for the understanding of the specific enzyme Cytosolic aspartate aminotransferase from horse heart, mapping its key properties and reactivities.
• This research might aid in the broader investigation of aspartate aminotransferase enzymes across different species, potentially revealing new structural or functional insights.
• Finally, the unique acetylation at the N-terminus of the horse heart enzyme may point to intriguing biological differences. These could provide a platform for further research into potential unique roles or mechanisms of action for the horse heart enzyme.