Lactoperoxidase consists of domains: a scanning calorimetric study

Lactoperoxidase folding and catalysis relies on the stabilization of the α-helix rich core domain: A thermal unfolding study

Lactoperoxidase (LPO) belongs to the mammalian peroxidase family and catalyzes the oxidation of halides, pseudo-halides and a number of aromatic substrates at the expense of hydrogen peroxide. Despite the complex physiological role of LPO and its potential involvement in carcinogenic mechanisms, cystic fibrosis and inflammatory processes, little is known on the folding and structural stability of this protein. We have undertaken an investigation of the conformational dynamics and catalytic properties of LPO during thermal unfolding, using complementary biophysical techniques (differential scanning calorimetry, electron spin resonance, optical absorption, fluorescence and circular dichroism spectroscopies) together with biological activity assays. LPO is a particularly stable protein, capable of maintaining catalysis and structural integrity up to a high temperature, undergoing irreversible unfolding at 70 degrees C. We have observed that the first stages of the thermal denaturation involve a minor conformational change occurring at 40 degrees C, possibly at the level of the protein beta-sheets, which nevertheless does not result in an unfolding transition. Only at higher temperature, the protein hydrophobic core, which is rich in alpha-helices, unfolds with concomitant disruption of the catalytic heme pocket and activity loss. Evidences concerning the stabilizing role of the disulfide bridges and the covalently bound heme cofactor are shown and discussed in the context of understanding the structural stability determinants in a relatively large protein.

Heat capacities of amino acids, peptides and proteins

The heat capacity is one of the fundamental parameters describing thermodynamic properties of a system. It has wide applications in a number of areas such as polymer chemistry, protein folding and DNA stability. To aid the scientific community in the analysis of such data, I have compiled a database on the experimentally measured heat capacities of amino acids, polyamino acids, peptides, and proteins in solid state and in aqueous solutions.

Molecular cloning of cDNAs encoding bovine and human lactoperoxidase

Peptide sequences obtained from cyanogen bromide fragments of bovine lactoperoxidase (bLPO) were used to design oligonucleotide probes for library screening. These probes were used to screen a cDNA library constructed from bovine mammary tissue. Three overlapping clones were obtained, the longest of which (T3) contained a reading frame of 712 amino acid residues. The encoded amino acid sequence was homologous to those recently reported for myelo-, thyro-, and eosinophil peroxidases. Two possible amino termini of the mature enzyme were identified, and the predicted mature protein matched previous molecular weight estimates of 78,500. Of eight bovine tissues tested, transcription of T3 sequences were detected in mammary tissue only. Using the bLPO cDNA as a probe, a single hybridizing clone was found in a human mammary gland cDNA library. This clone (M1) encoded the carboxy-terminal 324 residues of a peroxidase distinct from the other three known human peroxidases, and was closely related to bLPO. This result confirms the presence of at least one distinct lactoperoxidase in humans.

Domains in bovine serum amine oxidase

Analysis of the thermal unfolding of bovine serum amine oxidase by differential scanning calorimetry reveals for the dimeric protein a four domain structure consisting of two sets of domains. Each set contains two domains of similar size. The two smaller domains, in contrast with the larger ones, greatly differ in thermostability. Removal of copper changes the calorimetric pattern dramatically. The findings confirm that the metal cofactor plays a structural role. Since the enzyme contains two copper atoms and only one titratable carbonyl group, the calorimetric pattern suggests that the difference in thermostability of the two small domains might be due to the presence of a single organic cofactor.