Factor Xa

From Proteopedia

Revision as of 20:23, 25 April 2010

Introduction

Factor X is a vitamin K-dependent glycoprotein that is synthesized in the liver. Zymogen factor X circulates in plasma as a 2 chain molecule composed of a disulfide linked light chain (Mr = 16500) and heavy chain (Mr = 42,000). Factor X is activated to factor Xa by cleavage of the activation peptide. This reaction is catalyzed by factor VIIa-tissue factor (extrinsic Xase complex) and factor IXa-factor VIIIa (intrinsic Xase complex).^[1]

Factor Xa, along with factor Va, calcium, and a phospholipid membrane surface form the prothrombinase complex, to cleave prothrombin to its active form, thrombin.^[1]

Structure

The factor Xa light chain contains a γ-carboxyglutamic acid (Gla) domain (11 gla residues) as well as two epidermial growth factor (EGF)-like domains.^[2] The Gla domain is responsible for the high-affinity binding of calcium ions and interactions with phospholipid membrane surfaces. Recent crystal structures suggest that the N-terminal epidermal growth factor (EGF)-like domain is flexibly, while the second EGF domain maintains contacts with the catalytic domain ^[3]

Catalytic Triad

Substrate Recognition Sites

The S1 pocket determines binding selectivity for factor Xa. It is formed by loops in residues 214-220 and 189-195 that are linked by a Cys220-Cys191 disulfide bond. Residues 225-228 form the lower portion of the pocket.

The S2 site of factor Xa is formed by the 90s loop which is positioned adjacent to His57. Consistent with glycine as the P2 element in prothrombin, S2 is a small, shallow pocket.

S4 pocket is formed between the 90s and 170s loops and bind an Ile. This region contains 3 ligand binding domains. The hydrophobic box is located at the entrance to S4 and contains Phe174, Tyr99 and Trp215, which form a deep aryl-binding pocket. The cationic hole is formed by Glu97 and the backbone carbonyl of Lys96. The water site is composed of the hydrophobic side chains of Thr98, Ile175 and Thr177 and traps a water molecule. ^[4]

Activation domain

Enzyme Mechanism

General Serine Protease Mechanism

During the acylation half of the reaction Ser195 attacks the carbonyl of the peptide substrate, His57 assists by acting as a general base to yield a tetrahedral intermediate. Asp102 stabilizes the His57-H+ through hydrogen bonding. The tetrahedral intermediate oxyanion is stabilized by interacting with amine groups of the peptide backbone. Upon collapse of the tetrahedral the amine leaving group is expelled with the asstance of His57-H+ acting as a general acid to yield the acylenzyme intermediate. The deacylation portion repeats the same sequence. Water is assisted by His57 to attack the acyl enzyme, to yield another tetrahedral intermediate. Upon intermediate collapse Ser195 and the carboxylic acid product are expelled. ^[5]

Related Enzymes

References

1. ↑ ^{1.0 1.1} Greer, John (2008). Wintrobe's Clinical Hematology, p. 545-546. Lippincott Williams & Wilkins. ISBN 0781765072.
2. ↑ Padmanabhan K, Padmanabhan KP, Tulinsky A, Park CH, Bode W, Huber R, Blankenship DT, Cardin AD, Kisiel W. Structure of human des(1-45) factor Xa at 2.2 A resolution. J Mol Biol. 1993 Aug 5;232(3):947-66. PMID:8355279 doi:http://dx.doi.org/10.1006/jmbi.1993.1441
3. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named EGF_domain
4. ↑ Rai R, Sprengeler PA, Elrod KC, Young WB. Perspectives on factor Xa inhibition. Curr Med Chem. 2001 Feb;8(2):101-19. PMID:11172669
5. ↑ Hedstrom L. Serine protease mechanism and specificity. Chem Rev. 2002 Dec;102(12):4501-24. PMID:12475199