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Beta-Secretase (Memapsin 2, BACE-beta-site APP-cleaving enzyme, Asp2)

Alzheimer's disease (AD) is an atypical development of the brain that leads to the loss of physical and mental functions. While most victims of this dementing disorder are over 65 years of age, a small percentage of Alzheimer patients are under age 50. The Alzheimer and related dementia population in our nation is comprised of nearly 3 to 4 million people, with risks increasing with age [4]. While the cause of AD has not been fully elucidated, several breakthroughs have recently been made concerning its pathogenesis.

Proteases
A protease or peptidase is an enzyme involved in peptide bond hydrolysis [5]. Peptidases are further divided into two subcategories entitled endopeptidases and exopeptidases or proteinases. While endopeptidases cleave peptide bonds at positions within the protein, exopeptidases sequentially remove amino acids from the N or C-terminus. Categorized in relation to their catalytic mechanisms, four classes of proteinases have been recognized by the International Union of Biochemistry and Molecular Biology (IUBMB). They include: serine proteinases, cysteine proteinases, aspartic proteinases, and metalloproteinases. [5]

Aspartic Proteinases
For the purpose of this paper, our interest is in the aspartic proteases. The majority of the aspartic proteinases belong to the pepsin family. These are bilobed molecules that have their active site between the two homologous lobes. One aspartate residue is contributed by each lobe forming the catalytic diad of aspartates. Unlike serine and cysteine proteases in which a covalent intermediate is formed during catalysis, aspartic proteases form a tetrahedral intermediate. [5]

Human Aspartic Proteinases
Human aspartic proteases have key roles in biology and medicine. Five well-studied human aspartic proteases are 1) pepsin, 2) gastricsin, 3) cathepsin D and E, and 4) renin. Pepsin and gastricsin participate in digestion in the stomach. Cathepsin D and E, on the other hand, are involved in intracellular protein degradation in lysosomes and endosomes. Human cathepsin D has shown some correlation to breast cancer metastasis and to Alzheimer's disease. Finally, renin is involved in homeostasis as it catalyzes the conversion of angiotensin to angiotensin 1. [6]

Beta-secretase
The identification of the beta-secretase of Alzheimer's disease has long been sought. Hong et al. were the first to generate a crystal structure of beta-secretase complexed with its inhibitor in October of 2000. Around the same time, a wave of researchers jumped to grab claim for uncovering the true identity of this enzyme (also known as BACE beta-secretase site APP-cleaving enzyme, Asp2, and memapsin 2). A synthetic construct isolated from Homo sapiens, beta-secretase-the subject of this report-has been classified as a human aspartic protease and a class I transmembrane protein of 501 amino acids [7]. It consists of an NH2-terminal protease domain, a connecting strand, a transmembrane region, and a cytosolic [8,9,10] C-terminal tail [9]. Homologous to sequence of other aspartic proteases, beta-secretase contains a 48 residue pro sequence at its NH2-terminal region. Furthermore, it requires an acidic pH for proper activity. It is similar to pepsin in many aspects but different in the fact that it is significantly larger due to a 35 residue C-terminal extension and due to insertions. This enlargement enhances beta-secretase's association with other cellular components.

Activity
beta-secretase aids in the production of beta-amyloidamyloid peptide (alpha-beta from beta-amyloid precursor protein (APP). While initially thought to be the product of an aberrant process, studies have shown that in cells that express APP, alpha-beta secretion is a normal physiological occurrence [10,11,12]. The alpha-beta peptide is generated from both N and C termini cleavage of the alpha-beta domain within APP [2, 13,14]. The enzyme alpha-secretase cleaves within amyloid sequence [2], thus eliminating alpha-beta fragment release from APP. Contrastingly, gamma secretase cleaves APP within the transmembrane, at the C-terminus, and beta secretase cleaves APP on the lumenal side of the membrane, at the N-terminus, yielding alpha-beta peptide formation. ore specifically, beta secretase hydrolyzes peptides from the APP beta secretase site [15], cleaving the membrane-anchored APP at +1 and +11 sites [OMIM]. Beta-secretase's cleavage is the rate-limiting step of alpha-beta production in vivo [16]. The Endoplasmic Reticulum (ER) generated alpha-beta is a 40-42 residue protein that, upon accumulation, significantly attributes to the pathogenesis of Alzheimer's disease [17]. Because of their activities, both beta and gamma secretases are potential targets for inhibitor drugs against alpha-beta formation and hence, the proliferation of Alzheimer's disease. Furthermore, its transmembrane domain and unusual disulfide structures yield beta secretase as a prime target for AD treatment.

Structure
Located on gene11q 23.(see Mechanism), beta-secretase is a bilobal, 70 kD protein [18,19]. It has 501 amino acids (a.a.), including a 21 a.a. signal peptide, a 48 a.a. pro protein domain, a transmembrane region and a 24 a.a. C-terminal tail [20,7] (See Figure 1.). N-glycosylation sites are asn 153, asn 172, asn 223, asn 354 of the ectodomain. The ectodomain consists of 6 cys residues engaged in disulfide bridges/ intramolecular linkages between residues 216 and 420, 278 and 443, and 330 and 380 [20,13].

Figure 1