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Bio-medical Ontologies Maintenance and Change Management

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Substructure Analysis of Metabolic Pathways 247<br />

is to show that the substructures found by graph-based relational learning<br />

are biologically important <strong>and</strong> meaningful.<br />

5.1 Graph Representation<br />

Input graphs for SUBDUE are converted from KGML files. KGML is a st<strong>and</strong>ard<br />

data format to express <strong>and</strong> distribute a biological network from KEGG.<br />

There are three major entities in KGML: Entry, Relation <strong>and</strong> Reaction. Entry<br />

represents various biomolecules in the metabolic pathway, such as enzyme,<br />

gene, compound <strong>and</strong> so on. Relation denotes a relationship between two or<br />

more enzymes, genes <strong>and</strong> maps. The maps denote the types of the Entry<br />

nodes linked to the other pathways [26]. The names of these Entry nodes<br />

represent the name of the linked pathways. Reaction is a biochemical reaction<br />

between two or more compounds catalyzed by one or more enzymes.<br />

Detailed information on KGML is described in [26]. In biochemical semantics,<br />

Entries are nodes of metabolic pathways, <strong>and</strong> Relations <strong>and</strong> Reactions<br />

are relationships between two or more Entries.<br />

enzyme<br />

type<br />

type<br />

S_to_Rct<br />

entry<br />

reaction<br />

name<br />

ec:1.4.1.3<br />

name<br />

entry<br />

E_to_Rct<br />

type<br />

compound cpd:06560<br />

name<br />

reversible<br />

E_to_Rel<br />

Rct_to_P<br />

Rn:R05573<br />

GErel<br />

type<br />

type<br />

relation<br />

entry<br />

name<br />

subtype<br />

compound<br />

compound cpd:06562<br />

value<br />

Rel_to_E<br />

S_to_Rct<br />

reversible<br />

Rn:R05575<br />

type<br />

enzyme ec:1.4.1.5<br />

type<br />

entry<br />

name<br />

E_to_Rct<br />

type<br />

Fig. 3. A graph representation of a metabolic pathway<br />

reaction<br />

name<br />

Rct_to_P<br />

entry<br />

name<br />

compound cpd:06563<br />

In our graph representation, Relations <strong>and</strong> Reactions are also represented<br />

as vertices in order to describe the properties of Relations <strong>and</strong> Reactions.<br />

Vertices representing major entities have two satellite vertices which are connected<br />

to their main vertex by edges, labeled as Name <strong>and</strong> Type, to explain<br />

its property. A name vertex linked by the Name edge denotes the KEGG ID,<br />

<strong>and</strong> a type vertex linked by the Type edge describes the property of the entity<br />

vertex. A Relation represents the association between two or more Entries<br />

(genes or enzymes) by an edge whose label represents a direction from one

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