module: dmel_chitin_metabolism
title: Chitin biosynthesis and catabolism in Drosophila melanogaster
label: Chitin metabolism
module_type: METABOLIC_PATHWAY
description: >-
  Chitin, a beta-1,4-linked polymer of N-acetylglucosamine, is the fibrous
  scaffold of the insect cuticle, tracheal linings and the peritrophic matrix
  of the midgut. It is synthesized at the plasma membrane by chitin synthases:
  Krotzkopf verkehrt (kkv) makes chitin of the epidermal cuticle and trachea,
  and Chs2 is the second chitin synthase, expressed in the gut. During molting
  and tissue remodelling, chitin is broken down by family 18 chitinases (Cht
  genes), which cleave the polymer into oligosaccharides, and by
  beta-N-acetylhexosaminidases (Hexo1, Hexo2), which release
  N-acetylglucosamine from the non-reducing ends.
summary: >-
  Chitin synthesis by Kkv and Chs2, and chitin degradation by chitinases and
  beta-N-acetylhexosaminidases.
concepts:
  - {id: GO:0006030, label: chitin metabolic process}
parts:
  - id: chitin_synthesis
    label: Chitin synthesis
    role: UDP-N-acetylglucosamine polymerization into chitin
    genes: [kkv, Chs2]
    complex: false
    function: {id: GO:0004100, label: chitin synthase activity}
    processes:
      - {id: GO:0006031, label: chitin biosynthetic process}
  - id: chitin_hydrolysis
    label: Chitin hydrolysis by chitinases
    role: chitin to chito-oligosaccharides
    description: >-
      Family 18 chitinase genes grouped by FlyBase as chitin catabolism
      enzymes; individual activities and substrates have mostly not been
      measured.
    group: CHITCA
    exclude: [Hexo1, Hexo2]
    complex: false
    processes:
      - {id: GO:0006032, label: chitin catabolic process}
  - id: oligosaccharide_hydrolysis
    label: N-acetylglucosamine release
    role: chito-oligosaccharides to N-acetylglucosamine
    genes: [Hexo1, Hexo2]
    complex: false
    function: {id: GO:0004563, label: beta-N-acetylhexosaminidase activity}
    processes:
      - {id: GO:0006032, label: chitin catabolic process}
connections:
  - source: chitin_synthesis
    target: chitin_hydrolysis
    connection_type: PROVIDES_INPUT_FOR
    description: Chitin deposited by the synthases is the substrate of chitinases at molting.
  - source: chitin_hydrolysis
    target: oligosaccharide_hydrolysis
    connection_type: PRECEDES
    description: Chito-oligosaccharides released by chitinases are hydrolysed by hexosaminidases.
knowledge_gaps:
  - gap_statement: >-
      Which of the ten Cht genes encode active chitinases, and which tissues and
      molts each acts in, is largely unknown.
    boundary: Chitinase step of chitin catabolism.
    gap_kind: [BIOLOGY]
notes: >-
  Boundary. The module joins the FlyBase chitin biosynthesis (CHIYBI) and chitin
  catabolism (CHITCA) groups. UDP-N-acetylglucosamine synthesis, chitin
  deacetylases and chitin-binding cuticle proteins are not parts. Chitinase
  activity is not assigned to individual Cht genes because activity has not
  been shown for most of them. The fused lobes hexosaminidase is not a FlyBase
  member of the group and is not included.
groups: [CHIYBI, CHITCA]