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Polysaccharide Lyase Family 7
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|Polysaccharide Lyase Family PL7|
|3D Structure||β jelly roll|
|Active site residues||known|
|CAZy DB link|
Alginate lyases (Alys) of all families, PL5-7, PL14-15, PL17-18, catalyse the anionic alginate in three steps: (I)removal of the begative charge on teh carboxylate anion, (II)general base-catalysed abstraction of the proton on the C5 and (III) β-elimination of the 4-O-glycosidic bond . Most PL7s are endo-active, i.e. acting within a poly- or oligosaccharide and releasing smaller alginate fragments, while exo-actining PL7s are cleaving a monosaccharide from the polymer termini . In both activity modes, a new nonreducing end with a 4-deoxy-L-erythro-hex-4 en pyranosyl uronate residue (Δ) is formed. In contrast to terrestrial PL7, marine PLs need the divalent cation calcium for substrate recognition and binding . Ca2+ is weakening the ionic interactions between substrate (polyanion) and PL7 (polycation) by reducing the surface density of the alginate charge and therefore increasing the enzyme activity .
Kinetics and catalytic residues
Several structural and biochemical analyses of wild type and mutated PL7s revealed five residues forming the active site: arginine (R), glutamine (Q), histidine (H), tyrosine (Y), which are assembled in three highly conserved regions: R*ELR*ML, VIIGQIH, YFKAG*Y*Q (Figure 1). Osawa and colleagues assumed that in PL7 ALY-1 from Corynebacterium sp. Q117+Y195 interact near the reaction site of alginate to maintain proper orientation of the substrate, R72 interacts with alginate due to the formation of salt bridges with the carboxyl groups at the C5 and H119 acts as a base to deprotonate . However, there can also be additional charged residues at the active site, which promote substrate recognition and binding . Such residues can be found in the N-terminal R*ELREML and VIIGQIH regions. Both highly conserved regions are mainly characterized by hydrophobic amino acids (especially aromatic amino acids) such as leucine, tryptophan and methionine as well as residues with planar polar side chains (especially amino acids with charged side chains) such as arginine, glutamic acid, glutamine (Figure 2). These residues have been suggested to be substrate-binding molecules .
The polysaccharide lyase family 7 (PL7) contains five subfamilies (SF) based on their sequence similarities , plus a so far uncharacterized sixth subfamily, which consist only of marine representatives of the Flavobacteriaceae (Figure 2) . All characterized PL7 enzymes were alginate lyases specific for the anionic, gel forming polysaccharide alginate. The substrate specificity depends on the source of alginate, i.e. derived from brown seaweed or mucoid bacteria Pseudomonas spp. and Azotobacter vinelandii, as well as geographical and saisonal parameters. Alginate is an heteropolysaccharide, consisting of β-D-mannuronate (M) and α-L-guluronate (G). These monosaccharides can occur in homogenous and heterogenous blocks. Hence, PL7 lyases can be mannuronate (EC 22.214.171.124), guluronate (EC 126.96.36.199) or mixed link (EC 4.2.2.-) specific. Despite the preference for M- or G-enriched blocks, most PL7s also have a moderate to low processivity for the other building block ,,. polyG specific PL7s have been found in the SF3 and SF5  with QIH in the second highly conserved region, while polyM specific PL7s are characterised by QVH , .
PL 7 is a very well biochemical characterized family with almost 40 entries in the CAZy data base . Structural insights on the other hand are still restricted with nine 3D structures from only eight bacterial strains (status at CAZy in August 2019). The first structure of a PL7 was determined from Pseudomonas aeruginosa by multiple isomorphous replacement (MIR) at 2.0 Å resolution . Just like PL14, PL7 belongs to the jelly roll family with a wide open cleft harboring the active site (Figure 3A, B). Til date, there is only one known exoactive PL7 structure .. Zobellia galaactinovorans DsijT is harboring, among others, two PL7 with two completely different activity modifs. AlyA1 belongs to SF3 and is an endo-active PL7. AlyA5 on the other hand belongs to SF5 and is exo-active, which active site is close by three additional loops forming a small pocket (Figure 3C, D). The highly conserved 9-amino-acid-block YFKAGVY*Q, where * is a variable residue, at the C-terminus of PL7s (Figure 2) has also been found for an extracellular pectate lyase in E. chrysanthemi (Keen & Tamaki, 1986) . Alginate and pectate / pectin lyases share several features such as β-elimination, the recognition of substrates of a similar structure and primary sequence similarity, indicating that they probably share a similar core structural fold. Since alginate lyases and pectinases differ in substrate specificity, it is likely not related to substrate recognition, but rather to maintaining a stable 3D-conformation . Several alginate lyases have been reported to be multimodula domain enzymes with putative non-catalytic, carbohydrate-binding modules (CBMs). However, their exact role has been barely investigated yet. The first biochemically characterized CBM of an endo PL7 was a N-terminal CBM13 from Agarivorans sp. L11, which not only increased its substrate binding ability and therefore catalytic efficiency, but also substrate preference and product distribution as well as thermostability . Contradictory observations regarding catalytic effiency and substrate specificity were made for an endo PL7 from Vibrio splendidus OU02 DNA with an N-terminal CBM32 linked by a unique alpha helix linker . Nevertheless, the CBM and und linker were supposed to serve as "pivont point" affeting the product distribution towards trisaccharides. The PL7 from Persicobacter sp. CCB-QB2 is even consisting of three domains - a N-terminal CBM16 with still unclear function, C-terminal catalytic domain and a CBM32, which is located between both domains . This CBM32 is also not enhancing the catalytic activity and is not binding alginate, but the cleaved termini during catalysis. The crystal structures revealed an arginine residue which is possibly binding to the carboxylic group and a conserved Ca2+ binding site most likely essential for the maintenance of the overall fold.
Gene transfer of Alys among different habitats
Alginate degrading organisms often posses specified gene cluster for glycan utilization which contain, among other proteins such as transporters, several endo- and exo-actining Alys of different families. These gene clusters are called polysaccharide utilization loci (PULs) for Bacteriodetes  or alginolytic operons in case a SusCD pair transporter is missing or replaced by a different transporter system. It has been shown that these gene cluster can be transfered horizontally from one organism to another and thereby even cross different environmental habitats . The first alginate utilization system (AUS) was found in Zobellia galactinovorans which contains two clusters harboring five out of seven Alys (3 PL7s). Those operons origninated from an ancestral marine Flavobacterium and were independently transferred to marine Proteobacteria and Japanese gut Bacteriodetes by lateral gene transfer (LGT) .
- First catalytic endo-activity
polyM PL7 from Photobacterium ATCC 433367 
polyG PL7 from Klebsiella pneumoniae subbsp. aerogenes 
- First catalytic exo-activity
- AlyA5 from Zobellia galactanivorans DsijT 
- First 3-D apo-structure
- PA1167 from Pseudomonas aeruginosa 
- First 3-D holo-structure
- A1-II from Sphingomons sp. A1 
- First characterised CBM
- AlyL2 containing a N-terminal CBM13 from Agarivorans sp. L11 
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