View source for Carbohydrate Binding Module Family 21

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* [[Author]]s: ^^^Birte Svensson^^^ and ^^^Stefan Janecek^^^
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== Ligand specificities ==
Modules from family CBM21 bind to the α-glucan starch and oligosaccharides derived from starch or related to such oligosaccharides that contain α-1,4-linked glucose and/or α-1,6-linked glucose including maltose through maltoheptaose, β- and γ-cyclodextrins, isomaltotriose and isomaltotetraose <cite>Chou2006 Chu2014 Liu2007 Tung2008</cite>. CBM21 also interacts with amylose and alters its ultrastructure as demonstrated by atomic force microscopy <cite>Jiang2012</cite>. Circular permutation enhanced the affinity for amylose <cite>Stephen2012</cite>. The domain has been described as providing mainly glucoamylase and α-amylase with the ability to bind onto raw-starch (starch granules) <cite>Ashikari1986 Bui1996 Houghton-Larsen2003 Steyn1995 Kang2004</cite>.

== Structural Features ==
Structures of CBM21 have been determined by NMR and X-ray crystallography both in free and in carbohydrate complexed form. They adopt a common β-sandwich fold and have two binding sites accommodating carbohydrate ligands similarly to other starch binding domains. The binding sites contain aromatic side chains participating in carbohydrate interaction. Initially it was described by modelling using an NMR structure of a CBM20 as template <cite>Chou2006</cite> and thereafter by docking onto the NMR structure determined for CBM21 from the family [[GH15]] ''Rhizopus oryzae'' glucoamylase <cite>Liu2007</cite>. The crystal structure determined for this CBM21 in complex with β-cyclodextrin or maltoheptaose identified W47, Y83 and Y94 interacting at site I and Y32 and F58 at site II for both ligands <cite>Tung2008</cite>. Site I requires ligands with DP > 3 for binding <cite>Chu2014</cite>. A CBM21-like domain was identified in the crystal structure of barley family [[GH13]] limit dextrinase <cite>Moeller2012</cite>.

== Functionalities == 
CBM21s are mainly associated with some fungal glucoamylases and α-amylases of the family [[GH15]] <cite>Ashikari1986 Bui1996 Houghton-Larsen2003</cite> and [[GH13]] <cite>Steyn1995 Kang2004</cite>, respectively. This was confirmed by an exhaustive evolutionary analysis of 85 fungal genomes <cite>Chen2012</cite>. In both cases, i.e. in [[GH15]] glucoamylases and [[GH13]] α-amylases, the CBM21 precedes the catalytic domain <cite>Machovic2005</cite>. The CBM21 is present also as a part of the regulatory subunit of Ser/Thr-specific protein phosphatases that directs the protein phosphatase to glycogen <cite>Bork1998</cite>. Recently, a structural comparison identified an N-terminal CBM21-like domain in the barley GH13 limit dextrinase <cite>Moeller2012</cite>, where it is followed by the module from the family [[CBM48]] succeeded by the catalytic TIM-barrel.

== Family Firsts ==
;First Identified
Family CBM21 was first observed as an N-terminally located domain in glucoamylase from ''Rhizopus oryzae'' <cite>Ashikari1986</cite>. The function was ascribed based on comparison of multiple forms of the glucoamylase <cite>Ashikari1986 Takahashi1982</cite> and amino acid sequence alignment <cite>Tanaka1986</cite>. The CBM21 sequence was revealed as related to that of the starch binding domain of ''Aspergillus niger'' glucoamylase <cite>Svensson1989</cite>, which has been assigned the family [[CBM20]] <cite>Machovic2005 Machovic2006 Christiansen2009</cite>.
;First Structural Characterization
The first CBM21 three-dimensional structure was determined by NMR for the module from the family [[GH15]] glucoamylase from ''Rhizopus oryzae'' <cite>Liu2007</cite>. The first CBM21 complex structures were determined by X-ray crystallography for that domain binding to β-cyclodextrin or maltoheptaose <cite>Tung2008</cite>.

== Novel Applications ==
The CBM21 from ''Rhizopus oryzae'' glucoamylase has been introduced as a novel affinity purification tag <cite>Lin2009</cite>.      

== References ==
<biblio>
#Chou2006 pmid=16509822
#Chu2014 pmid=24108499
#Liu2007 pmid=17117925
#Tung2008 pmid=18588504
#Jiang2012 pmid=22815939
#Stephen2012 pmid=23226294
#Ashikari1986 Ashikari T, Nakamura N, Tanaka Y, Kiuchi N, Shibano Y, Tanaka T, Amachi T, and Yoshizumi H. “Rhizopus raw-starch-degrading glucoamylase: its cloning and expression in yeast.” Agric. Biol. Chem. 1986; 50: 957-64.
#Bui1996 pmid=8920185
#Houghton-Larsen2003 pmid=12883866
#Steyn1995 pmid=8529895
#Kang2004 pmid=15043869
#Moeller2012 pmid=22949184
#Chen2012 pmid=23166747
#Machovic2005 pmid=16262690
#Bork1998 pmid=9500672
#Takahashi1982 pmid=6818228
#Tanaka1986 Tanaka Y, Ashikari T, Nakamura N, Kiuchi N, Shibano Y, Amachi T, and Yoshizumi H. Comparison of amino acid sequences of three glucoamylases and their structure-function relationships. Agric. Biol. Chem. 1986; 50: 965-9.
#Svensson1989 pmid=2481445
#Machovic2006 pmid=17013558
#Christiansen2009 pmid=19682075
#Lin2009 pmid=19297701      
</biblio>

[[Category:Carbohydrate Binding Module Families|CBM021]]