Abstract
Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.
Highlights
Type-3 copper proteins have two copper ions and three histidines (H) in each active site pocket (Ashida and Brey, 1997; Aguilera et al, 2013)
Mammalian tyrosinase activity is closely related to skin and hair color, and loss of tyrosinase activity in humans is the direct cause of albinism and leucoderma (Oetting and King, 1999; Kirkwood, 2009)
Arthropod hemocyanin mainly transfers oxygen in the hemolymph, while tyrosinase in microbes is positively related to pathogenicity (Mayer, 2006; Shang et al, 2012) and, in insects and other arthropods, PPO is an important innate immunity protein (Ashida and Brey, 1997; Cerenius et al, 2008; Kanost and Gorman, 2008)
Summary
Type-3 copper proteins have two copper ions and three histidines (H) in each active site pocket (Ashida and Brey, 1997; Aguilera et al, 2013). Type-3 copper proteins from different organisms are named differently; e.g., tyrosinase in mammals and microbes, prophenoloxidase (PPO) in insects and crabs, polyphenol oxidase ( termed as PPO) in plants, and hemocyanin in arthropods (Cerenius et al, 2008; Aguilera et al, 2013). Arthropod hemocyanin mainly transfers oxygen in the hemolymph (van Holde and Miller, 1995), while tyrosinase in microbes is positively related to pathogenicity (Mayer, 2006; Shang et al, 2012) and, in insects and other arthropods, PPO is an important innate immunity protein (Ashida and Brey, 1997; Cerenius et al, 2008; Kanost and Gorman, 2008). PPO is a humoral protein that can induce melanization around invading pathogens after activation, and induces cellular and humoral immunity simultaneously
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