Dipeptidyl peptidase IV

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1 Dipeptidyl Peptidase IV

Dipeptidyl 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.

Introduction

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^[1], 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. ^[2] 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.^[3]

DPP IV also serves as a binding glycoprotein on the membrane of cells, binding ligands such as adenosine deaminase with high affinity.^[3] Though this interaction currently has no known significance, DPP IV and its ability to catalyze N-terminal prolines gives it a unique specificity and target for pharmaceutical companies. ^[1]

Structure

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. ^[3] 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. ^[3]

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.^[3] 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 more nucleophilic.

General arrow pushing mechanism for serine proteases.
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 covalently 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 oxyanion intermediate, another important function in enzymatic processes. ^[4] The active site in its entirety is considered to contain residues 39-51 and 501-766 and is known as the

Lastly, the (colored by monomer) of DPP IV is critical to the catalytic function. The hydrolase domain and the propeller domain are involved in the dimerization of DPP IV. Residues like provide hydrophobic interactions that are essential for forming the dimer. ^[5] Though whole domains play key roles in the formation of this dimer, single residues like have also been shown to be key to formation of the dimer. If point mutated to glutamate, the dimer will not form. The ionic interactions of the histidine with the opposing chain are changed from electrostatically positive to a repulsive effect thus eliminating the ability to dimerize. ^[3]

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). The 8-bladed β-propeller of DPP IV is unique among the more common 7-bladed propellers of other leucocyte surface molecules.^[3] 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, with three important residues being , at the fourth and fifth propeller. Adenosine deaminase works to deaminate adenosine into inosine, an important function in purine metabolism^[1], however it's most important role in humans deals with the immune system.^[3] ADA is a well understood enzyme that is highly conserved across numerous species in the body^[3], yet it's binding to DPP IV is not completely understood, especially the biological importance of the interaction. 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. ^[3]

Medical 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. ^[3] The ability of DPP IV to inactivate incretins glucagon-like-peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) have made it a potential 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 very rapidly (the half-lives of GLP-1 and GIP are less than two minutes). ^[6] DPP IV inhibitors prevent DPP IV from inactivating GLP-1 and GIP, which results in improved glucose tolerance, improved 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. ^[7] , also known as Januvia, is a DPP IV inhibitor that's in the incretin mimetic class. It was approved by the FDA for the treatment of type II diabetes in 2006. When given to control subjects, Sitagliptin increases plasma concentrations of GLP-1. Sitagliptin was approved to be used in combination with metformin in 2007. ^[8] A study done by Williams-Herman et. al showed that initial combination therapy with metformin and sitagliptin resulted in lower blood glucose concentrations at each metformin dose studied than mono therapy of metformin. A majority of the patients in the combination therapy group maintained the desired blood glucose concentration of <7% at the end of the two-year trial period. ^[9] This is noteworthy because it's more difficult for people to meet their glycemic goals long term as the disease progresses. Sitagliptin was approved to be administered with sulfonylureas in 2008, which are insulin-secreting agents. Combination therapy of these two drugs requires close monitoring and adjustment of the sulfonylurea dose to prevent hypoglycemia ^[10]. Statins, like simvastatin, which help lower cholesterol levels, are also frequently prescribed to patients with Type II diabetes in addition to DPP IV inhibitors like sitagliptin. ^[10]

3D structures of dipeptidyl peptidase IV

Dipeptidyl peptidase

References

↑ ^1.0 ^1.1 ^1.2 Mentlein R. Dipeptidyl-peptidase IV (CD26)--role in the inactivation of regulatory peptides. Regul Pept. 1999 Nov 30;85(1):9-24. PMID:10588446
↑ Lankas GR, Leiting B, Roy RS, Eiermann GJ, Beconi MG, Biftu T, Chan CC, Edmondson S, Feeney WP, He H, Ippolito DE, Kim D, Lyons KA, Ok HO, Patel RA, Petrov AN, Pryor KA, Qian X, Reigle L, Woods A, Wu JK, Zaller D, Zhang X, Zhu L, Weber AE, Thornberry NA. Dipeptidyl peptidase IV inhibition for the treatment of type 2 diabetes: potential importance of selectivity over dipeptidyl peptidases 8 and 9. Diabetes. 2005 Oct;54(10):2988-94. PMID:16186403
↑ ^3.00 ^3.01 ^3.02 ^3.03 ^3.04 ^3.05 ^3.06 ^3.07 ^3.08 ^3.09 ^3.10 Gorrell MD. Dipeptidyl peptidase IV and related enzymes in cell biology and liver disorders. Clin Sci (Lond). 2005 Apr;108(4):277-92. PMID:15584901 doi:http://dx.doi.org/10.1042/CS20040302
↑ Bjelke JR, Christensen J, Branner S, Wagtmann N, Olsen C, Kanstrup AB, Rasmussen HB. Tyrosine 547 constitutes an essential part of the catalytic mechanism of dipeptidyl peptidase IV. J Biol Chem. 2004 Aug 13;279(33):34691-7. Epub 2004 Jun 2. PMID:15175333 doi:http://dx.doi.org/10.1074/jbc.M405400200
↑ Chien CH, Tsai CH, Lin CH, Chou CY, Chen X. Identification of hydrophobic residues critical for DPP-IV dimerization. Biochemistry. 2006 Jun 13;45(23):7006-12. PMID:16752891 doi:http://dx.doi.org/10.1021/bi060401c
↑ Green BD, Flatt PR, Bailey CJ. Dipeptidyl peptidase IV (DPP IV) inhibitors: A newly emerging drug class for the treatment of type 2 diabetes. Diab Vasc Dis Res. 2006 Dec;3(3):159-65. PMID:17160910 doi:http://dx.doi.org/10.3132/dvdr.2006.024
↑ Lambeir AM, Durinx C, Scharpe S, De Meester I. Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV. Crit Rev Clin Lab Sci. 2003 Jun;40(3):209-94. PMID:12892317 doi:10.1080/713609354
↑ Green BD, Flatt PR, Bailey CJ. Dipeptidyl peptidase IV (DPP IV) inhibitors: A newly emerging drug class for the treatment of type 2 diabetes. Diab Vasc Dis Res. 2006 Dec;3(3):159-65. PMID:17160910 doi:http://dx.doi.org/10.3132/dvdr.2006.024
↑ Williams-Herman D, Johnson J, Teng R, Golm G, Kaufman KD, Goldstein BJ, Amatruda JM. Efficacy and safety of sitagliptin and metformin as initial combination therapy and as monotherapy over 2 years in patients with type 2 diabetes. Diabetes Obes Metab. 2010 May;12(5):442-51. doi:, 10.1111/j.1463-1326.2010.01204.x. PMID:20415693 doi:http://dx.doi.org/10.1111/j.1463-1326.2010.01204.x
↑ ^10.0 ^10.1 Scheen AJ. Dipeptidylpeptidase-4 inhibitors (gliptins): focus on drug-drug interactions. Clin Pharmacokinet. 2010 Sep;49(9):573-88. doi: 10.2165/11532980-000000000-00000. PMID:20690781 doi:http://dx.doi.org/10.2165/11532980-000000000-00000