Mitochondrial dNTP supply for mtDNA maintenance — MNGIE and mtDNA depletion syndromes

Faithful replication of mitochondrial DNA (mtDNA), including in post-mitotic tissues, requires a balanced supply of the four deoxyribonucleoside triphosphates (dNTPs) inside mitochondria. Because mitochondria cannot rely on the cell-cycle-regulated cytosolic de novo machinery, they use a dedicated salvage pathway plus a p53-inducible de novo input, and depend on cytosolic enzymes that keep nucleoside pools balanced. The mitochondrial deoxyribonucleoside kinases phosphorylate imported deoxyribonucleosides to dNMPs: thymidine kinase 2 (TK2) handles the pyrimidines (thymidine, deoxycytidine, deoxyuridine) and deoxyguanosine kinase (DGUOK) the purines (deoxyguanosine, deoxyadenosine). The p53-inducible ribonucleotide reductase small subunit RRM2B (p53R2), with RRM1, provides de novo deoxyribonucleoside diphosphates (dNDPs) by reducing NDPs — the main dNTP source for quiescent cells and mtDNA. Cytosolic thymidine phosphorylase (TYMP) governs systemic thymidine/deoxyuridine levels; its loss floods the dNTP pool with thymidine-derived nucleotides and destabilises mtDNA. Together these enzymes maintain the mitochondrial dNTP balance; imbalance (excess or deficiency) causes mtDNA depletion/deletions. Inherited defects are the classic mtDNA-maintenance disorders: TYMP -> MNGIE; TK2 -> myopathic mtDNA depletion; DGUOK -> hepatocerebral mtDNA depletion; RRM2B -> encephalomyopathic mtDNA depletion and dominant progressive external ophthalmoplegia.

MODULE:mitochondrial_dntp_supplyDRAFTMetabolic Pathwaymodules/mitochondrial_dntp_supply.yaml
deoxyribonucleotide biosynthetic processGO:0009263
GO:0009263
deoxyribonucleotide biosynthetic process
The module groups the enzymes that supply/balance the mitochondrial dNTP pool (deoxyribonucleotide biosynthetic process, GO:0009263) for mtDNA replication.
GO:0006264
mitochondrial DNA replication
The physiological endpoint served is mitochondrial DNA replication (GO:0006264); dNTP imbalance from defects in these enzymes causes mtDNA depletion/instability.
file:human/TK2/TK2-ai-review.yaml
TK2 gene review (human)
The mitochondrial pyrimidine deoxyribonucleoside-salvage step (UniProtKB:O00142, GO:0004797) matches the completed human TK2 review.
file:human/DGUOK/DGUOK-ai-review.yaml
DGUOK gene review (human)
The mitochondrial purine deoxyribonucleoside-salvage step (UniProtKB:Q16854, GO:0004138) matches the completed human DGUOK review.
file:human/RRM2B/RRM2B-ai-review.yaml
RRM2B gene review (human)
The p53-inducible de novo dNDP-supply step (UniProtKB:Q7LG56, GO:0004748) matches the completed human RRM2B review.
file:human/TYMP/TYMP-ai-review.yaml
TYMP gene review (human)
The thymidine-pool-regulating step (UniProtKB:P19971, GO:0009032) matches the completed human TYMP review.
5Nodes
4Parts
0Variant Sets
0Variants
4Annotons
0Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:mitochondrial_dntp_supply deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Function consistency and core coverage

4 gene-function check(s) reflected in core functions · 0 conflict(s) · 4 evidence or coverage gap(s) (8 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
tk2_activity TK2 (human)
UniProtKB:O00142
Family Representative 		thymidine kinase activity
GO:0004797 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
dguok_activity DGUOK (human)
UniProtKB:Q16854
Family Representative 		deoxyguanosine kinase activity
GO:0004138 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
rrm2b_activity RRM2B (human, p53R2)
UniProtKB:Q7LG56
Family Representative 		ribonucleoside-diphosphate reductase activity, thioredoxin disulfide as acceptor
GO:0004748 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tymp_activity TYMP (human)
UniProtKB:P19971
Family Representative 		thymidine phosphorylase activity
GO:0009032 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tk2_activity Deoxyribonucleoside kinase family (TK2/DGUOK/DCK)
PANTHER:PTHR10513
Family 		thymidine kinase activity
GO:0004797 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR10513.
dguok_activity Deoxyribonucleoside kinase family (TK2/DGUOK/DCK)
PANTHER:PTHR10513
Family 		deoxyguanosine kinase activity
GO:0004138 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR10513.
rrm2b_activity Ribonucleotide reductase small subunit family (RRM2/RRM2B)
PANTHER:PTHR23409
Family 		ribonucleoside-diphosphate reductase activity, thioredoxin disulfide as acceptor
GO:0004748 		Family Scope Restricted
GO:0004748 is SUBFAMILY_ONLY; this family grounding does not establish the allowed subfamily scope. The activity the dNTP and mitochondrial dNTP-supply modules assert for the RNR role. It is safe, with the contributes_to sense appropriate for a subunit, for the radical-bearing small subunits in SF18, SF20 and SF19. It does not apply to the R2lox oxidases (SF36), and it is unestablished for SF21 (F54H12.2). Within SF18, yeast Rnr4 is catalytically inactive. It still contributes to holoenzyme activity as the obligate beta' partner of Rnr2, and its gene review keeps the activity term in that sense, so it is not listed as an exception here. A module selector "PTHR23409 = radical-bearing small subunit" therefore also captures Rnr4.
tymp_activity Thymidine/pyrimidine-nucleoside phosphorylase family (TYMP)
PANTHER:PTHR10515
Family 		thymidine phosphorylase activity
GO:0009032 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR10515.

Gene-review completeness (4/4 grounded genes reviewed)

4 complete review(s) · 0 with deep research · 0 missing review · 4 reviewed but lacking deep research

Gene Review Complete Deep research
DGUOK Q16854 ✓ ✓ ✗
RRM2B Q7LG56 ✓ ✓ ✗
TK2 O00142 ✓ ✓ ✗
TYMP P19971 ✓ ✓ ✗

Details

Context
mitochondrionGO:0005739
Mitochondrial dNTP supply (mtDNA maintenance)Metabolic Pathwaymitochondrial_dntp_supply
deoxyribonucleotide biosynthetic processGO:0009263
Context
mitochondrionGO:0005739

Mitochondrial dNTP-supply / mtDNA-maintenance system grounded to the human enzymes TK2 (UniProtKB:O00142, GO:0004797, EC 2.7.1.21), DGUOK (Q16854, GO:0004138, EC 2.7.1.113), RRM2B (Q7LG56, GO:0004748, EC 1.17.4.1) and TYMP (P19971, GO:0009032, EC 2.4.2.4). GO molecular-function terms were taken from the human GOA records; Reactome reaction ids and titles were verified against the local reactome cache. Each node uses a PANTHER family selector with the human enzyme as representative (TK2 and DGUOK are paralogs in the same deoxyribonucleoside-kinase family PTHR10513, with the cytosolic DCK/TK1 relatives; RRM2B is the p53-inducible paralog of the cell-cycle RRM2). This is a CONVERGENT-supply module rather than a linear pathway: TK2 (pyrimidine) and DGUOK (purine) salvage imported deoxyribonucleosides to dNMPs inside the matrix; RRM2B/RRM1 supply de novo dNDPs (cytosolic, imported); and TYMP keeps the thymidine/deoxyuridine pool balanced. The dNMPs/dNDPs are further phosphorylated (CMPK/NME kinases) to the dNTPs used by the mtDNA replisome (POLG). Their shared endpoint is a balanced mitochondrial dNTP pool for mtDNA replication (GO:0006264); genetic imbalance in any of them causes an mtDNA-maintenance disorder: TYMP -> MNGIE; TK2 -> myopathic mtDNA depletion (MTDPS2); DGUOK -> hepatocerebral mtDNA depletion (MTDPS3); RRM2B -> encephalomyopathic mtDNA depletion (MTDPS8) and autosomal-dominant progressive external ophthalmoplegia. RRM2B's dNTP supply and TYMP's angiogenic (PD-ECGF) moonlighting are captured in the respective gene reviews.

Part 1: mitochondrial pyrimidine deoxyribonucleoside salvage
thymidine / deoxycytidine / deoxyuridine + ATP to dTMP / dCMP / dUMPReactiontk2_step

Annotons

TK2: mitochondrial thymidine kinase 2
tk2_activity
Participant: Family: Deoxyribonucleoside kinase family (TK2/DGUOK/DCK)
Family:
Deoxyribonucleoside kinase family (TK2/DGUOK/DCK)PANTHER:PTHR10513
Representative Members: TK2 (human)UniProtKB:O00142

Function

thymidine kinase activityGO:0004797
Substrates: thymidine / deoxycytidine / deoxyuridine ATP
Products: dTMP / dCMP / dUMP ADP

Locations

mitochondrial matrixGO:0005759

Pyrimidine arm of the mitochondrial deoxyribonucleoside salvage pathway, phosphorylating thymidine/deoxycytidine/deoxyuridine to dNMPs for mtDNA. Deficiency = myopathic mtDNA depletion syndrome (MTDPS2).

Part 2: mitochondrial purine deoxyribonucleoside salvage
deoxyguanosine / deoxyadenosine + ATP to dGMP / dAMPReactiondguok_step

Annotons

DGUOK: mitochondrial deoxyguanosine kinase
dguok_activity
Participant: Family: Deoxyribonucleoside kinase family (TK2/DGUOK/DCK)
Family:
Deoxyribonucleoside kinase family (TK2/DGUOK/DCK)PANTHER:PTHR10513
Representative Members: DGUOK (human)UniProtKB:Q16854

Function

deoxyguanosine kinase activityGO:0004138
Substrates: deoxyguanosine / deoxyadenosine / deoxyinosine ATP
Products: dGMP / dAMP / dIMP ADP

Locations

mitochondrial matrixGO:0005759

Purine arm of the mitochondrial deoxyribonucleoside salvage pathway, phosphorylating deoxyguanosine/deoxyadenosine to dNMPs for mtDNA. Deficiency = hepatocerebral mtDNA depletion syndrome (MTDPS3).

Part 3: de novo dNDP supply (p53-inducible ribonucleotide reductase)
ribonucleoside diphosphate (NDP) to 2'-deoxyribonucleoside diphosphate (dNDP)Reactionrrm2b_step

Annotons

RRM2B (p53R2): ribonucleotide reductase small subunit
rrm2b_activity
Participant: Family: Ribonucleotide reductase small subunit family (RRM2/RRM2B)
Family:
Ribonucleotide reductase small subunit family (RRM2/RRM2B)PANTHER:PTHR23409
Representative Members: RRM2B (human, p53R2)UniProtKB:Q7LG56

Function

ribonucleoside-diphosphate reductase activity, thioredoxin disulfide as acceptorGO:0004748
Substrates: ribonucleoside diphosphates (ADP/GDP/CDP/UDP) thioredoxin (reductant)
Products: 2'-deoxyribonucleoside diphosphates (dNDPs)

Locations

cytosolGO:0005829

p53-inducible small subunit of ribonucleotide reductase (with RRM1) providing the de novo dNDP supply for DNA repair and for mtDNA synthesis in quiescent cells. Deficiency = encephalomyopathic mtDNA depletion (MTDPS8) / dominant PEO.

Part 4: thymidine-pool regulation (protects dNTP balance)
thymidine + phosphate <-> thymine + 2-deoxyribose-1-phosphateReactiontymp_step

Annotons

TYMP: thymidine phosphorylase
tymp_activity
Participant: Family: Thymidine/pyrimidine-nucleoside phosphorylase family (TYMP)
Family:
Thymidine/pyrimidine-nucleoside phosphorylase family (TYMP)PANTHER:PTHR10515
Representative Members: TYMP (human)UniProtKB:P19971

Function

thymidine phosphorylase activityGO:0009032
Substrates: thymidine / 2'-deoxyuridine phosphate
Products: thymine / uracil 2-deoxy-alpha-D-ribose 1-phosphate

Locations

cytosolGO:0005829

Cytosolic control of thymidine/deoxyuridine levels; its loss (MNGIE) floods the nucleotide pool with thymidine/deoxyuridine, imbalancing mitochondrial dNTPs and destabilising mtDNA.