Asparaginase

Function

L-Asparaginases are enzymes that hydrolyze the amide group of the amino acid L-asparagine (L-Asn) to L-aspartate, with the simultaneous release of ammonia. They are often referred to as ASNase and assigned the EC number 3.5.1.1; if significant glutaminase co-activity (hydrolysis of the similar amino acid L-glutamine) is also present, the EC number is 3.5.1.38. Some ASNases that belong to other classes (see below) are assigned EC 3.4.19.5.

Relevance

Beyond pure academic curiosity, ASNases are also studied because of their potential application as first-line drugs for the treatment of acute lymphoblastic leukemia (ALL) [1-4]^[1][2][3][4]. By clearing L-asparagine from circulation, they starve the L-Asn-dependent malignant cells to death, while sparing the L-Asn-independent healthy cells. The first L-asparaginase introduced for clinical treatment of ALL was Elspar (EcAII from E. coli), followed by Erwinase (ErA from E. chrysanthemi). ASNases are also used in food industry to prevent the formation of acrylamide from L-Asn in fried starch foods^[5][6].

Division of ASNases into three classes

So far, three completely different structural classes of ASNases have been identified^[7], originally named according to the source organism of their isolation^[8], namely Class 1 (bacterial-type), Class 2 (plant-type), and Class 3 (Rhizobium etli-type). This newer classification is intersected with an older convention, which divided the known enzymes with L-asparaginase activity into five types, since in both, Class 1 and Class 3, two types are distinguished according to their compartmentalization and expression profile. The prototypes of types I and II (in Class 1), and III (in Class 2), are the E. coli enzymes EcAI (cytosolic), EcAII (periplasmic), and EcAIII (also cytosolic), respectively. The prototypes of types IV and V (Class 3) are the R. etli enzymes ReAIV (constitutive) and ReAV (inducible).

Structural studies

The first L-asparaginase structure was published and deposited in the PDB in 1993 for the EcAII enzyme [9] and may serve as an example of a Class 1 type II enzyme (Figure EcAII_3eca). Structure of Class 1 type I enzymes is exemplified by EcAI [10] (Existing Figure), whereas Class 2 type III enzymes may be represented by EcAIII [11, 12] (Figure EcAIII_2zal). Class 2 L-asparaginases belong to the family of Ntn-hydrolases, which are expressed as inactive precursors that must undergo autoproteolytic cleavage into α and β subunits to achieve maturation [13]. While the existence of an alien type of ASNase in the symbiotic nitrogen-fixing bacterium Rhizobium etli had been recognized long ago [14], the structure of the inducible and thermolabile prototype Class 3 ReAV was solved and deposited in the PDB only recently [15] (Figure ReAV_7os5), followed by structures of the constitutive and thermostable isoform ReAIV [16]. More than 200 structures of ASNases have been deposited in the Protein Data Bank (PDB) by April 2024 [17] (Wlodawer et al., 2024).

Structural highlights

.^[9] Ligands asparagine (in white) and aspartic acid (in salmon) are shown in spacefill representation.

• , chain A. Water molecules shown as red spheres.
• , chain A.
• .

3D structures of asparaginase

Asparaginase 3D structures