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This enzyme catalyzes the interconversion of glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), an important step in glycolysis. The active enzyme is a dimer composed of identical subunits. Each monomer is composed of a barrel of parallel beta-sheets with interconnecting alpha helices. This structure can be clearly seen in the accompanying image.
The enzyme functions by promoting proton exchange to form an enediol and then stabilizes this intermediate, thus lowering the energy of activation. The active site has a glutamic acid residue (GLU 165) and a histidine residue (HIS 95) to allow for this proton shuttle. Other important active site residues are GLU 97 and LYS 13.
To view the active site for triose phosphate isomerase click the "binding pocket" box on the lower right hand corner of the accompanying clickable image, or click anywhere in the active site itself. This image shows the enzyme dimer bound to phosphoglycolohydroxatate (PGH). PGH is a transition state analog for rhe enediol intermediate, and functions as a potent inhibitor of trios phosphate isomerase. By mimicing the structure of the enediol intermediate PGH is bound more tightly to the enzyme than either G3P or DHAP, the natural substrates.
The human immunodeficiency virus (HIV) causes acquired immune deficiency syndrome (AIDS). The virus is a retrovirus, meaning that its genome consists of single stranded RNA. The enzyme HIV reverse transcriptase catalyzes the conversion of HIV viral RNA to HIV viral DNA. The enzyme is packaged in virions and enters the infected cell aling with the viral RNA. Reverse transcriptase fuctions by using the viral RNA as a template for synthesising a complementary DNA strand, with this DNA then inserting into the host's chromosomal DNA.
AZT, or 3'-azido-3'-deoxythymidine, functions as an antiviral nucleoside analog. When anabolized to the corresponding 5' triphosphate it inhibits reverse transcriptase, thus halting the production of viral DNA. Residues in the active site of the enzyme,however, soon mutate and reverse transcriptase becomes AZT resistant.
The accompanying image shows the tertiary structure of HIV-I reverse transcriptase. To obtain an enlarged view of the AZT active site and the region of mutation click the box labelled "residues of the active site mutated in AZT resistance" in the upper left hand corner. Dipyridodiazepinone nevirapine is another antiviral nucleoside analog with a different site of action, and an enlarged view of the drug and the active site can be obtained by clicking either of the other labelled boxes in the upper left hand corner.
Examples of enzymes in this class include :
These enzymes are called proteases because they catalyze the hydrolysis of peptide bonds in polypeptides and proteins. They are termed serine proteases due to the fact that all enzymes in this class have a serine residue in the active site that plays a critical role in the catalytic process. Most serine proteases have similar three dimensional structures and share some common active site features.
The active site for each of these enzymes includes an aspartic acid residue, a histidine and a serine residue. In chymotrypsin these are ASP 102, HIS 57 and SER 195. These residues form a catalytic triad that mediate a charge relay process. The HIS 57 accepts a proton from the SER 195, as the serine initiates a nucleophilic attack on the carboxyl group of the substrate.
Click here to view an image of this catalytic triad.
Carboxypeptidase A is a zinc-containing metalloenzyme that catalyzes the C-terminal peptide bond of protein substrates. These metal catalysed hydrolysis is to be contrasted with the general base catalysis of the serine proteases
