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<StructureSection load='1aay' size='350' side='right' | <StructureSection load='1aay' size='350' side='right' caption='' scene=''> | ||
==Motifs In Proteins== | ==Motifs In Proteins== | ||
The term "motif" when used in structural biology tends to refer to one of two cases: | The term "motif" when used in structural biology tends to refer to one of two cases: | ||
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The example structure shown to <scene name='40/401510/Cv/3'>illustrate the motif</scene> is that of Zif268 protein-DNA complex from Mus musculus (PDB entry 1AAY). In this example (a C2H2 class zinc finger) the conserved <scene name='User:James_D_Watson/Structural_Templates/Zinc_finger_cysteine/1'>cysteine</scene> and <scene name='User:James_D_Watson/Structural_Templates/Zinc_finger_histidine/2'>histidine</scene> residues form ligands to a <scene name='User:James_D_Watson/Structural_Templates/Zinc_finger_zn/1'>zinc ion</scene> whose coordination is essential to stabilise the tertiary fold of the protein. The fold is important because it helps orientate the <scene name='User:James_D_Watson/Structural_Templates/Zinc_finger_recognition/1'>recognition helices</scene> to bind to the <scene name='User:James_D_Watson/Structural_Templates/Zinc_finger_major_groove/1'>major groove of the DNA</scene>. | |||
The example structure shown to <scene name=' | |||
{{Clear}} | {{Clear}} | ||
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Serine proteases are found in a number of organisms but common to their function is the hydrolysis of peptide bonds. These enzymes catalyse the reaction using a highly reactive serine residue to attack the carbonyl group of the backbone to be hydrolysed. The chemistry of this reaction and the regeneration of the active site, requires the presence of the Ser-His-Asp catalytic triad. In chymotrypsin (PDB entry [[1ab9]]) these residues are (Ser-195, His-57 and Asp-102) whereas in the bacterial subtilisin (PDB entry [[1st2]]) the site is formed by (Ser-221, His-64 and Asp-32). These two proteins are evolutionary unrelated and this is the classic example of convergent evolution to solve the problem of peptide bond hydrolysis. | Serine proteases are found in a number of organisms but common to their function is the hydrolysis of peptide bonds. These enzymes catalyse the reaction using a highly reactive serine residue to attack the carbonyl group of the backbone to be hydrolysed. The chemistry of this reaction and the regeneration of the active site, requires the presence of the Ser-His-Asp catalytic triad. In chymotrypsin (PDB entry [[1ab9]]) these residues are (Ser-195, His-57 and Asp-102) whereas in the bacterial subtilisin (PDB entry [[1st2]]) the site is formed by (Ser-221, His-64 and Asp-32). These two proteins are evolutionary unrelated and this is the classic example of convergent evolution to solve the problem of peptide bond hydrolysis. | ||
The detection of these types of motif is almost impossible by looking at the amino acid sequence: there is no evolutionary relationship to detect, the residues are ordered differently in the sequence, and the spacing between the residues also varies. These motifs can be detected relativeley easily using structural comparison, particularly template-based motif detection algorithms. | The detection of these types of motif is almost impossible by looking at the amino acid sequence: there is no evolutionary relationship to detect, the residues are ordered differently in the sequence, and the spacing between the residues also varies. These motifs can be detected relativeley easily using structural comparison, particularly template-based motif detection algorithms. Note that the global folds of subtilisin and chymotrypsin are very different so the site could not have been detected using such methods. Click to see the catalytic triad in <scene name='User:James_D_Watson/Structural_Templates/Subtilisin_startpoint_catalyti/1' target='subtilisin'>subtilisin</scene> and <scene name='User:James_D_Watson/Structural_Templates/Chymotrypsin_start_triad/1' target='chymotrypsin'>chymotrypsin</scene> respectively. | ||
< | </StructureSection> | ||