Journal:Acta Cryst D:S205979832500292X

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<StructureSection load='' size='450' side='right' scene='underdevelopment' caption=''>
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<StructureSection load='' size='450' side='right' scene='10/1078092/014_dh_page_opening/1' caption='The double-helical packing observed in PvAIII is particularly remarkable among proteins.'>
===Unique double-helical packing of protein molecules in the crystal of K-independent L-asparaginase from common bean===
===Unique double-helical packing of protein molecules in the crystal of K-independent L-asparaginase from common bean===
<big>Dr Joanna Loch</big> <ref>doi: 10.1107/S205979832500292X</ref>
<big>Dr Joanna Loch</big> <ref>doi: 10.1107/S205979832500292X</ref>
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<b>Molecular Tour</b><br>
<b>Molecular Tour</b><br>
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L-Asparaginases are enzymes that catalyze the hydrolysis of L-asparagine into L-aspartic acid and ammonia. This reaction is crucial for nitrogen metabolism in plants, as L-asparagine serves as a major nitrogen storage and transport molecule. Plant (Class 2) L-asparaginases are classified into two main types based on their dependency on potassium (K) ions: K-dependent and K-independent enzymes. K-dependent L-asparaginases require the presence of potassium ions for their enzymatic activity.
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In common bean (Phaseolus vulgaris), there are two K-dependent L-asparaginases, PvAIII(K)-1 and PvAIII(K)-2, encoded by the PvAspG1 and PvAspG2 genes, respectively. These enzymes play a significant role in nitrogen assimilation and are regulated by environmental factors such as light. K-independent L-asparaginases, on the other hand, do not require potassium ions for their activity.
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The genome of common bean encodes one K-independent L-asparaginase, PvAIII, expressed by the PvAsp-T2 gene. Its <scene name='10/1078092/014_fig_2b/1'>3D structure</scene> of K-independent L‑asparaginase PvAIII was determined at 1.88 Å.resolution. The enzyme shows a higher affinity for β-peptides than for L-asparagine, suggesting that its physiological role may be more related to detoxification processes rather than basic L-asparagine metabolism. This discovery highlights the importance of PvAIII in managing toxic protein degradation products in plants.
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The K-independent L-asparaginase (PvAIII) from the common bean exhibits an extraordinary crystal structure (PDB ID: 9HNC). This structure is characterized by a rare P2 space-group symmetry and a unique pseudosymmetric double-helical packing, containing 32 protein chains in the asymmetric unit. The structure's uniqueness arises from the ability of the PvAIII molecule to form extensive intermolecular β-sheets, the incomplete degradation of the interdomain flexible linker, and the presence of intermolecular hydrogen bonds that connect adjacent protein chains.
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Packing of PvAIII molecules in the crystal shows an unusual double-helical arrangement. This comes about as the <scene name='10/1078092/014_fig_1a_txt/1'>asymmetric unit</scene> contains two coiled arrangements of chains A-H (green shades) and I-P (magenta shades). The crystallographic translation along [001] generates the <scene name='10/1078092/014_fig_1b_txt/1'>first strand of superhelix</scene> A-H and the first strand of superhelix I-P. The <scene name='10/1078092/014_fig_1c_new_txt/1'>second strands of both superhelices</scene> are generated by the crystallographic twofold axis along [010]. It may be easier to see the <scene name='10/1078092/014_fig_1c_new_txt/2'>2 double-helices via rotation</scene>.
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<b>References</b><br>
<b>References</b><br>
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</StructureSection>
</StructureSection>
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The double-helical packing observed in PvAIII is particularly remarkable among proteins.

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