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AK2 is the adenylate kinase isozyme localized to the mitochondrial intermembrane space, distinct from the matrix-localized AK3/AK4, supporting the intermembrane-space localization annotation.
"AK2 is repeatedly described as the **adenylate kinase isozyme localized to the mitochondrial intermembrane space (IMS)** and distinguished from **AK3/AK4**, which are generally described as mitochondrial matrix enzymes."
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AK2 generates ADP in the intermembrane space to supply mitochondrial ATP synthesis, coupling to adenine nucleotide exchange via the adenine nucleotide translocator (ANT).
"A key functional framing is that **AK2 generates ADP in the IMS**, thereby helping supply ADP for mitochondrial ATP synthesis and coupling to adenine nucleotide exchange (e.g., via the adenine nucleotide translocator, ANT)."
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For zebrafish Ak2 the best-supported molecular function is adenylate kinase activity (EC 2.7.4.3) acting on ATP and AMP to produce ADP, rather than a broad nucleoside diphosphate kinase activity.
"For functional annotation of zebrafish Ak2 (Q1L8L9), the best-supported specific assignment from the available evidence remains **adenylate kinase activity (EC 2.7.4.3) acting on ATP and AMP to produce ADP** in the mitochondrial IMS."
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Adenylate kinases can use other purine or pyrimidine NTPs as phosphate donors in the generalized reaction, but AMP is always the phosphoryl acceptor and AMP/ATP remain the canonical EC 2.7.4.3 substrates.
"Canonical substrates are **AMP plus ATP** for the EC 2.7.4.3 reaction; AK enzymes can also use other **purine or pyrimidine NTPs** as phosphate donors in the generalized reaction, while **AMP is the acceptor**."
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In zebrafish ak2 transcripts are maternally inherited and decline after the onset of zygotic transcription.
"In zebrafish, **ak2 transcripts are maternally inherited** and decrease after the onset of zygotic transcription (qRT-PCR described)."
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Whole-mount in situ hybridization localizes zebrafish ak2 expression to hematopoietic sites, including the intermediate cell mass at ~22-24 hpf and the caudal hematopoietic tissue at 5 dpf.
"Whole-mount in situ hybridization (WISH) localizes **ak2 expression to hematopoietic sites**, including the **intermediate cell mass (ICM)** around ~22–24 hours post-fertilization and the **caudal hematopoietic tissue (CHT)** at 5 days post-fertilization."
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Loss of ak2 impairs definitive hematopoiesis across multiple zebrafish alleles and morpholino perturbations.
"Across alleles and perturbations, **ak2 deficiency impairs definitive hematopoiesis**"
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ak2-mediated mitochondrial adenine nucleotide homeostasis protects hematopoietic development from ROS-associated oxidative stress, providing the mechanistic link from enzyme function to the hematopoietic phenotype.
"These findings support a mechanistic model in which AK2-mediated adenine nucleotide homeostasis in mitochondria protects hematopoietic development from ROS-associated stress."
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The zebrafish L124P missense allele lies in a region connecting the CORE and LID domains and is predicted to impair protein stability and enzymatic activity, supporting the conserved adenylate kinase fold.
"the zebrafish **L124P** missense allele is positioned in a region described as connecting the **CORE** and **LID** domains and is predicted to impair protein stability and enzymatic activity."