Glycoside Hydrolase Family 22

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• Author: ^^^Spencer Williams^^^
• Responsible Curator: ^^^David Vocadlo^^^

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Glycoside Hydrolase Family GH22
Clan	none, lysozyme-type fold
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/GH22.html

Substrate specificities

Glycoside hydrolase family 22 contains proteins with two main functions: lysozymes and α-lactalbumin.

Lysozymes catalyse the hydrolysis of (1→4)-β-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in peptidoglycan and between N-acetyl-D-glucosamine residues in chitooligosaccharides. Lysozymes are also referred to as muramidases. Lysozymes from family GH22 are classified as c-type lysozymes (c = chicken), to distinguish them from lysozymes of family GH23, which are sometimes referred to as g-type (g = goose) lysozymes. Lysozymes provide a range of functions that are related to their bacteriolytic action. As secretions in milk, saliva, mucus, and tears, and their presence in egg-white, they protect against bacterial infection through their ability to degrade the bacterial cell wall. As intestinal secretions in ruminants such as the cow (Bos taurus) they assist in lysis of commensal gut bacteria, releasing their nutrients for the host. Human lysozyme defects can result in a rare hereditary condition, amyloidosis VIII, in which lysozyme deposits as amyloid [1].

α-Lactalbumins are auxiliary proteins that bind to and modify the substrate specificity of galactosyltransferase (a family GT7 enzyme that in the absence of α-lactalbumin transfers glucose to N-acetylglucosamine), converting it to lactose synthase (i.e. catalyzing transfer to glucose). It is believed that α-lactalbumins evolved at the outset of mammalian evolution, after divergence of mammalian and avian lineages [2, 3]. α-Lactalbumin expression is induced by prolactin, and occurs within the mammary glands.

Kinetics and Mechanism

HEWL operates through a classical Koshland retaining mechanism involving a covalent glycosyl enzyme intermediate. Evidence in support of this mechanism was obtained by detection of a covalent glycosyl enzyme intermediate through the use of the substrate chitobiosyl fluoride and the HEWL E35Q (acid/base) mutant, detected by mass spectrometry; use of the modified substrate N-acetylglucosaminyl-(1,4)-2-deoxy-2-fluoroglycosyl fluoride and the HEWL E35Q mutant, andd detected by mass spectrometry and studied by X-ray crystallography [4]. Notably, a mechanism involving neighboring group participation was ruled out by showing that substrates bearing hydrogen and hydroxyl substitutions at 2-position are hydrolysed at similar rates to that of the parent compound bearing the acetamido group [5, 6], and thus that the 2-acetamido functionality is not critical for catalysis. HEWL catalyzed the hydrolysis of a series of aryl chitobiosides with varying leaving group ability with similar K_M values, but varying k_cat values, giving support for a rate-determining step involving concerted acid-base or acid-nucleophilic catalysis [7]. The HEWL-catalyzed hydrolysis of phenyl chitobioside afforded secondary kinetic isotope effects for H1/D1 of k_H/k_D 1.11, revealing considerable oxocarbenium ion character for the glycosylation transition state of the enzyme-catalyzed reaction [5].

α-Lactalbumins possess a conserved Ca²⁺ binding site, with a high affinity for the cation of 3-5 nM [8].

Catalytic Residues

Inspection of complexes of lysozyme with chitooligosaccharides and chemical reasoning led to the proposal of Glu35 as a proton donor [9]. Site directed mutagenesis of Glu35 to Gln35 resulted in a complete loss of activity against Micrococcus luteus cell wall [10]. Together these data support the identity of Glu35 as the general acid/base in a classical Koshland retaining mechanism. In an early study Asp52 was highlighted as a catalytic residue, and proposed to play a role in stablizing an oxocarbenium ion intermediate [9]. The Asp52Asn mutant exhibited approximately 5% wild-type lytic ability against Micrococcus luteus cell wall [10]. Asp52 is believed to function as a catalytic nucleophile, as shown by X-ray crystallographic observation of a covalent bond for the 2-fluoroglycosyl enzyme formed on the E35Q mutant of HEWL using N-acetylglucosaminyl-(1,4)-2-deoxy-2-fluoroglycosyl fluoride, and by mass spectrometric observation of a covalent adduct of the same complex [4].

α-Lactalbumins typically lack the conserved catalytic residues present in lysozymes. Two naturally occurring variants of human lysozyme, Ile56Thr and Asp67His, are amyloidogenic [1]. In both cases, decreased protein stability is believed to contribute to amyloid formation, with fibrils forming more readily at low pH or at slightly elevated temperatures.

Three-dimensional structures

The first structure of a GH22 member was that of hen egg white lysozyme (HEWL) [11, 12]. In fact, HEWL has a distinguished history as the first enzyme for which atomic resolution X-ray data was reported, and has attracted great interest as it provided the first molecular view of enzyme catalysis. An extensive range of lysozyme structures have been determined, including hundreds of structures of mutants, such that lysozyme is the most commonly deposited protein in the Protein Databank. Lysozyme adopts a compact globular structure comprised of just 127 amino acids. There are five helical regions comprising around 40% of the amino acids. There are also five regions of beta sheet with both random coil and beta turns. A large cleft runs across the face of the structure that comprises the active site and contains the catalytic residues Glu35 and Asp52. Four disulfide bonds are present in the structure: Cys6-Cys1127, Cys30-Cys115, Cys64-Cys80, and Cys76-Cys94. {discussion of complexes}

The structure of α-lactalbumin is essentially identical in three-dimensional fold to HEWL. The first α-lactalbumin to have a high resolution structure determined was that from baboon [13]. Baboon lactalbumin shares conserved disulfide bonds and a large cleft equivalent to the substrate binding cleft in HEWL. α-Lactalbumin possesses a Ca²⁺ binding subsite, which in the baboon enzyme is a distorted pentagonal bipyramid comprised of Lys79, Asp82, Asp84, Asp87 and Asp88 and two water molecules. X-ray structures of complexes of murine α-lactalbumin bound to bovine galactosyltransferase have been determined [14]. The interface is comprised mainly of hydrophobic interactions. Binding of α-lactalbumin results in a large conformational change in galactosyltransferase.

The lysozyme fold is shared by family GH19 chitinases, GH23 lysozymes, GH124 cellulases, and GH134 mannanases.

Family Firsts

First stereochemistry determination

Retention of glycosyl transfer to N-acetylglucosamine and methanol [15].

First catalytic nucleophile identification

Asp52 of hen egg white lysozyme (HEWL), by X-ray crystallography of covalent complex formed with a 2-fluorosugar [4].

First general acid/base residue identification

Glu35 of HEWL proposed on the basis of X-ray structure of a complex with a chitooligosaccharide [9]; the HEWL Glu35Gln mutant displayed a loss of activity against bacterial cell wall [10].

First 3-D structure

Hen egg-white lysozyme (HEWL) was the first glycosidase, and the first enzyme, to have its three-dimensional structure determined by X-ray diffraction techniques [12].

References

1. Jeyashekar NS, Sadana A, and Vo-Dinh T. (2005). Protein amyloidose misfolding: mechanisms, detection, and pathological implications. Methods Mol Biol. 2005;300:417-35. DOI:10.1385/1-59259-858-7:417 |
2. Prager EM and Wilson AC. (1988). Ancient origin of lactalbumin from lysozyme: analysis of DNA and amino acid sequences. J Mol Evol. 1988;27(4):326-35. DOI:10.1007/BF02101195 |
3. Qasba PK and Kumar S. (1997). Molecular divergence of lysozymes and alpha-lactalbumin. Crit Rev Biochem Mol Biol. 1997;32(4):255-306. DOI:10.3109/10409239709082574 |
4. Vocadlo DJ, Davies GJ, Laine R, and Withers SG. (2001). Catalysis by hen egg-white lysozyme proceeds via a covalent intermediate. Nature. 2001;412(6849):835-8. DOI:10.1038/35090602 |
5. Dahlquist FW, Rand-Meir T, and Raftery MA. (1969). Application of secondary alpha-deuterium kinetic isotope effects to studies of enzyme catalysis. Glycoside hydrolysis by lysozyme and beta-glucosidase. Biochemistry. 1969;8(10):4214-21. DOI:10.1021/bi00838a045 |
6. Rand-Meir T, Dahlquist FW, and Raftery MA. (1969). Use of synthetic substrates to study binding and catalysis by lysozyme. Biochemistry. 1969;8(10):4206-14. DOI:10.1021/bi00838a044 |
7. Lowe G, Sheppard G, Sinnott ML, and Williams A. (1967). Lysozyme-catalysed hydrolysis of some beta-aryl di-N-acetylchitobiosides. Biochem J. 1967;104(3):893-9. DOI:10.1042/bj1040893 |
8. Mitani M, Harushima Y, Kuwajima K, Ikeguchi M, and Sugai S. (1986). Innocuous character of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid and EDTA as metal-ion buffers in studying Ca2+ binding by alpha-lactalbumin. J Biol Chem. 1986;261(19):8824-9. | Google Books | Open Library
9. Blake CC, Johnson LN, Mair GA, North AC, Phillips DC, and Sarma VR. (1967). Crystallographic studies of the activity of hen egg-white lysozyme. Proc R Soc Lond B Biol Sci. 1967;167(1009):378-88. DOI:10.1098/rspb.1967.0035 |
10. Malcolm BA, Rosenberg S, Corey MJ, Allen JS, de Baetselier A, and Kirsch JF. (1989). Site-directed mutagenesis of the catalytic residues Asp-52 and Glu-35 of chicken egg white lysozyme. Proc Natl Acad Sci U S A. 1989;86(1):133-7. DOI:10.1073/pnas.86.1.133 |
11. BLAKE CC, FENN RH, NORTH AC, PHILLIPS DC, and POLJAK RJ. (1962). Structure of lysozyme. A Fourier map of the electron density at 6 angstrom resolution obtained by x-ray diffraction. Nature. 1962;196:1173-6. DOI:10.1038/1961173a0 |
12. Blake CC, Koenig DF, Mair GA, North AC, Phillips DC, and Sarma VR. (1965). Structure of hen egg-white lysozyme. A three-dimensional Fourier synthesis at 2 Angstrom resolution. Nature. 1965;206(4986):757-61. DOI:10.1038/206757a0 |
13. Acharya KR, Stuart DI, Walker NP, Lewis M, and Phillips DC. (1989). Refined structure of baboon alpha-lactalbumin at 1.7 A resolution. Comparison with C-type lysozyme. J Mol Biol. 1989;208(1):99-127. DOI:10.1016/0022-2836(89)90091-0 |
14. Ramakrishnan B and Qasba PK. (2001). Crystal structure of lactose synthase reveals a large conformational change in its catalytic component, the beta1,4-galactosyltransferase-I. J Mol Biol. 2001;310(1):205-18. DOI:10.1006/jmbi.2001.4757 |

15. Rupley JA, Gates V. Studies on the enzymic activity of lysozyme, II. The hydrolysis and transfer reactions of N-acetylglucosamine oligosaccharides. Proc. Natl. Acad. Sci. U.S.A. 1967; 57(3):496-510. [1] pmcid=PMC335536