{"id":"bbf474e2-87d4-5884-9bb5-d81bfca41c0e","stable_key":"649e861b-265a-5912-bc36-3a73e53ef892:l-serine-sepsecs-substrate","predicate":"requires","statement":"Human SepSecS structural and functional analysis supported correct positioning of tRNA-bound phosphoserine, but not free phosphoserine, for PLP-dependent selenocysteine formation.","claim_class":"mechanistic","status":"source_derived_draft","evidence_grade":"ungraded","direction":"context_dependent","is_public":true,"mechanism_event_id":"9fba2de8-77e2-5630-b052-a3caa0fbaa7a","mechanism_event_label":"The carrier and the cofactor are part of the reaction, not optional packaging.","subject":{"id":"192979cc-02fb-5913-bace-5d5ba3983f51","slug":"sepsecs","display_name":"SEPSECS / SepSecS","entity_type_key":"protein"},"object":{"id":"5b2eb213-ab47-5a57-871a-e00a5ef02990","slug":"phosphoseryl-trna-sec","display_name":"O-phosphoseryl-tRNA Sec","entity_type_key":"rna"},"evidence_count":1,"mechanism_event":{"id":"9fba2de8-77e2-5630-b052-a3caa0fbaa7a","stable_key":"649e861b-265a-5912-bc36-3a73e53ef892:l-serine-sepsecs-substrate-event","event_type":"observed_relationship","label":"The carrier and the cofactor are part of the reaction, not optional packaging.","description":"Human SepSecS structural and functional analysis supported correct positioning of tRNA-bound phosphoserine, but not free phosphoserine, for PLP-dependent selenocysteine formation.","status":"provisional","compartment":null,"participants":[{"entity":{"id":"192979cc-02fb-5913-bace-5d5ba3983f51","slug":"sepsecs","display_name":"SEPSECS / SepSecS","entity_type_key":"protein"},"role":"subject","stoichiometry":null,"state_label":"","sequence_order":0,"notes":""},{"entity":{"id":"5b2eb213-ab47-5a57-871a-e00a5ef02990","slug":"phosphoseryl-trna-sec","display_name":"O-phosphoseryl-tRNA Sec","entity_type_key":"rna"},"role":"target","stoichiometry":null,"state_label":"","sequence_order":1,"notes":""},{"entity":{"id":"c882cd0b-9d83-5b07-bd1c-fe4d45657dcb","slug":"l-serine","display_name":"L-Serine","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":2,"notes":""},{"entity":{"id":"6b656ff5-9532-5da4-8eea-8ca163a48649","slug":"pyridoxal-phosphate","display_name":"PLP","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":3,"notes":""},{"entity":{"id":"533add43-f6ba-5428-ab2d-74a45c054655","slug":"selenocysteine","display_name":"Sec","entity_type_key":"small_molecule"},"role":"context_participant","stoichiometry":null,"state_label":"","sequence_order":4,"notes":""}]},"contexts":[{"dimension":"evidence_access","value_text":"Primary abstract","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"experimental_model","value_text":"Human SepSecS–tRNA complex structure with enzyme assays.","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"limitations","value_text":"Free phosphoserine and a phosphoserine residue on this specialized tRNA are not interchangeable substrates.","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"nutrient_topic","value_text":"L-Serine collection; species, compartment, exposure, co-substrates and manipulation remain explicit.","comparator":null,"unit":null,"notes":"","entity":{"slug":"l-serine","display_name":"L-Serine","entity_type_key":"small_molecule"}},{"dimension":"plain_language","value_text":"The carrier and the cofactor are part of the reaction, not optional packaging.","comparator":null,"unit":null,"notes":"","entity":null},{"dimension":"primary_references","value_text":"The human SepSecS-tRNASec complex reveals the mechanism of selenocysteine formation. · 2009 · https://pubmed.ncbi.nlm.nih.gov/19608919/ · DOI 10.1126/science.1173755","comparator":null,"unit":null,"notes":"","entity":null}],"evidence":[{"id":"1e38eec7-0e1a-51df-b3ad-c6ac75e90c8f","evidence_kind":"source_excerpt","locator":"Lines 390-396","start_line":390,"end_line":396,"excerpt":"## l-serine-sepsecs-substrate\nThe carrier and the cofactor are part of the reaction, not optional packaging.\nHuman SepSecS structural and functional analysis supported correct positioning of tRNA-bound phosphoserine, but not free phosphoserine, for PLP-dependent selenocysteine formation.\nModel: Human SepSecS–tRNA complex structure with enzyme assays.\nLimitations: Free phosphoserine and a phosphoserine residue on this specialized tRNA are not interchangeable substrates.\nEvidence access: Primary abstract\nThe human SepSecS-tRNASec complex reveals the mechanism of selenocysteine formation. · 2009 · https://pubmed.ncbi.nlm.nih.gov/19608919/ · DOI 10.1126/science.1173755","model_system":"Human SepSecS–tRNA complex structure with enzyme assays.","directness":"reported_statement","verification_status":"source_derived_draft","notes":"Original curation paraphrase; evidence access and experimental limitations specified.","relationship":"supports","weight":1.0,"link_notes":"","source":{"id":"70bcf57a-36bb-563f-9073-2d616e5f155a","stable_key":"import-649e861b-265a-5912-bc36-3a73e53ef892","title":"L-Serine: synthesis, one-carbon metabolism, lipids and cross-nutrient mechanisms (2026-09-19)","document_type":"imported_text","citation_label":"AI-assisted research curation; primary references, access levels and experimental limitations individually identified. 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