Sandbox Reserved 918

This Sandbox is Reserved from Jan 06, 2014, through Aug 22, 2014 for use by the Biochemistry II class at the Butler University at Indianapolis, IN USA taught by R. Jeremy Johnson. This reservation includes Sandbox Reserved 911 through Sandbox Reserved 922.

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Dipeptidyl Peptidase IVDipeptidyl Peptidase IV

Surface representation of DPP-IV designed in PyMol from PDB 1X70. Red and blue represent individual chains of dimer with the ligand bound in yellow.

IntroductionIntroduction

Dipeptidyl Peptidase IV (commonly abbreviated as DPP IV or CD26) is a regulatory protease and binding glycoprotein that carries out numerous functions in humans. DPP IV, discovered by V.K. Hopsu-Havu and G.G. Glenner in homogenized rat liver tissue, was originally believed to serve a specific role in breaking 2-Naphthylamine off of Gly-Pro-2-napthylamide, hence its original name glycylproline napthylamidase. However, further research into the specificity of DPP IV showed that it serves a more generic function as a hydrolase (a serine exopeptidase), primarily cleaving N-terminal Xaa-Pro bonds, but also capable of cleaving N-terminal Xaa-Ala bonds. DPP IV is the founding member of the DPP-IV and/or structure homologue (DASH) family, who all share this serine protease catalysis of post-proline peptide bonds. These penultimate prolines of the N-terminus are known for their ability to resist attacks from most proteases and also induce a conformational change in their respective proteins, making DPP IV a unique protease for its ability to cleave this site.

DPP IV also serves as a binding glycoprotein on the membrane of cells, binding ligands such as adenosine deaminase with high affinity. Though this interaction currently has no known significance as of yet, DPP IV and its ability to catalyze N-terminal prolines gives it a unique specificity and target for pharmaceutical companies.

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StructureStructure

Binding Pocket The specificity of the DPP IV for proline from other amino acids can be seen in the where two glutamates, orient the substrate allowing only small residues like proline or alanine to fit. The substrate shown here, , is a common drug used to inhibit DPP IV. The glutamates form a salt bridge with the N-terminus, positioning the substrate so that only two amino acids can fit into position for hydrolysis. Examples of DPP IV substrates with alanine or proline at their N-terminus are: Substrate N-terminus Function GLP-1 His-Ala-Glu- Increases insulin secretion; decreases glucagon secretion GIP His-Ala-Asp- Increases insulin secretion; decreases glucagon secretion NPY Tyr-Pro-Ser- Vasoconstrictor; growth of fat tissue CXCL12 Lys-Pro-Val Chemotaxis of lymphocytes; angiogenesis Table 1: Common DPP IV substrates and with Xaa-Pro or Xaa-Ala bonds and their respective functions. Active Site These substrates, along with many others, are cleaved by DPP IV using an active site containing a catalytic triad composed of . This Serine-Histidine-Asparatate motif, best known in the enzyme chymotrypsin, uses acid-base chemistry to facilitate the binding, cleaving, and release of the given substrate. The mechanism of the reaction is as follows: Substrate binds to enzyme and carbonyl carbon is positioned by active site. Histidine, via hydrogen bond with asparatate, becomes more electronegative and therefore readily accepts the hydrogen from the hydroxyl group on serine, making it nucleophilic. [[Image:Serine_protease_mechanism_by_snellios.png|right|thumb|150px| [Arrow Pushing Mechanism}} ] The nucleophilic serine attacks the carbonyl carbon, generating a tetrahedral intermediate (as seen in the arrow pushing mechanism). The peptide bond is cleaved and the electrons from it move to attack the hydrogen on the histidine. This half of the substrate now dissociates, leaving the other half still bound to serine. Histidine then deprotonates a water molecule, forming a nucleophilic hydroxide group that attacks the serine-bound carbonyl carbon. As the electrons move from the oxygen back down to reform the double bond, serine dissociates as a leaving group and the other half of the substrate dissociates from the enzyme. The negative oxygen on serine readily accepts the hydrogen from histidine and in doing so regenerates the active site of the enzyme. In addition to the glutamates holding the substrate in close proximity, and the catalytic triad using acid base chemistry to cleave the peptide bond, there is a tyrosine, , which is depicted in orange and notably only 4.08 angstroms away from the substrate, Sitagliptin. Based off of the crystal structure, this tyrosine is believed to stabilize the tetrahedral intermediate, another important function in enzymatic processes. The active site in its entirety is considered to contain residues 39-51 and 501-766 and is known as the Lastly, the homodimerization of DPP IV is critical to the catalytic function. Though there are domains that play key roles in the formation of this dimer, a particular histadine () has been shown to inhibit the formation of the dimer if point mutated to glutamate. From this information it could be extrapolated that the histadine is forming some ionic interaction with the opposing chain, with the mutation from positive charge to negative charge creating a repulsive effect thus eliminating the ability to dimerize. Propeller Domain Though DPP IV's primary function is as a hydrolase, it also serves as a transmembrane glycoprotein on the surface of cells. A specific domain, the , works in binding the most well known DPP IV ligand, adenosine deaminase (or ADA). ADA can bind to either the monomer or dimer of DPP IV because each monomer contains the 8-bladed propeller domain. ADA actually binds to the lower side of this domain, at the fourth and fifth propeller. Adenosine deaminase works to deaminate adenosine into inosine, an important function in purine metabolism, however it's most important role in humans deals with the immune system. ADA is a well understood enzyme that is highly conserved across numerous species in the body, yet it's binding to DPP IV is not completely understood and there is no known reason as to why it occurs. One theory is that binding ADA to the DPP IV glycoprotein inhibits its catalytic function, increasing the concentration of extracellular adenosine which plays a role in T-cell proliferation.

spinqualitylabelsall models Biological Dimer of DPP IV Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Medical RelevancyMedical Relevancy

DPP IV is found in diverse tissue types and is involved in various biological functions. The activity of DPP IV has been studied in fields like immunology, endocrinology, and the biology of cancers. The ability of DPP IV to inactivate incretins glucagon-like-peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) have made it a well-studied protein because of its potential as a drug target for the treatment of Type II Diabetes. GLP-1 and GIP promote glucose uptake, decrease the gastric emptying rate and inhibit glucagon secretion. These actions are all desired when it comes to treating Type II diabetes, but the problem is that DPP IV inactivates GLP-1 and GIP rapidly (the half-lives of GLP-1 and GIP are less than two minutes). DPP IV inhibitors prevent DPP IV from inactivating GLP-1 and GIP, which results in improved glucose tolerance and pancreatic islet cell function, and a decrease in blood glucose levels. The decrease in blood glucose is associated with increased levels of active circulating GLP-1 and a reduction of glucagon. , also known as Januvia, is a DPP IV inhibitor that's well on its way to being approved for use in a plethora of countries. When given to control subjects, Sitagliptin increases plasma concentrations of GLP-1. Sitagliptin might be used in the future to help manage Type II Diabetes in combination with metformin.