Physiochemical Properties Molecular Weight

Most reports on the molecular weight of PPO are based on estimates on partially purified preparations using gel filtration or acrylamide gel electrophoresis. Therefore, a wide range of values are reported. Often crude or partially purified preparations show a multiplicity of forms, which may have resulted from association-dissociation reactions. Depending on the source of the enzyme, the molecular weights vary between 29,000 and 200,000 with subunit molecular weight from 29,000 to 67,000. Until recently, the enzyme from the bacterium Streptomyces glaucescens was found to have the smallest functional unit with one copper pair per polypeptide chain of 29,000 (44). The molecular weight of mushroom PPO has been reported to be between 116,000 and 128,000 daltons (37). The enzyme consisted of four identical subunits of Mw ca 30,000, each containing one copper atom (45). Two types of polypeptide chains, heavy (Mw 43,000) and light (Mw 13,400) were reported in mushroom (46). Avocado, banana, tea leaf, and other sources also showed multiple molecular weights in a wide range. The predominant form of PPO in grape had a molecular weight of 55,000 to 59,000 but upon storage it dissociated into 31,000-33,000 and 20,000-21,000 subunits (47). Based on recent knowledge regarding multiplicity of PPO, it is believed that many previous observations on multiplicity were due to secondary reactions, such as tanning, during enzyme preparation.

With the advent of recombinant DNA technology in recent years, numerous amino acid sequences have become available. These include PPO from the bacterium Strep-tomyces glaucescens (48) and Streptomyces antibioticus (49), tomato (50,51), broad bean (52), and potato (53). All PPO molecules sequenced are single-chain proteins with calculated Mw of 30,900 (273 amino acid residues) for S. glaucescens and S. antibioticus, 46,000 (407 amino acid residues) for N. crassa, 68,000 (606 amino acid residues) for the broad bean Vicia faba, and 66,300 (587 amino acid residues) for tomato.

The molecular weight of laccase was also reported to vary according to their sources. Rhus vernicifera laccase has been reported to vary between 101,000 and 140,000 depending on the species and variety. The Mw of the fungal laccase varies between 56,000 and 390,000; peach laccase was reported to have a Mw of 73,500. These data suggest that laccases are a heterogeneous group of glycoproteins, having a basic structural unit of between 50,000 and 70,000 Mw, which can undergo aggregation to give larger units.

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600 Chocolate Recipes

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