Glycoproteins Haemoproteins Medical Chemistry Lecture 15 2007 (J.S.)
hexoses (mannose, galactose),
N - acetylhexosamines (GlcNAc, GalNAc),
pentoses (arabinose, xylose),
6-deoxyhexoses (methylpentoses, e.g. L- fucose ),
sialic acids ,
in proteoglycans also glycuronic acids (GlcUA, GalUA, L- IdoUA)
Glycoproteins are glycosylated proteins , i.e. proteins that comprise a saccharide component attached to amino acid side chains by glycosidic bond. In most glycoproteins, the size of the saccharide components is in the range 1 – 15 % of molecular mass . A special type of glycoproteins are proteoglycans present in the extracellular matrix of connective tissue, in which the saccharide component may be larger than 90 % of molecular mass. Saccharide components of glycoproteins may comprise
Saccharide components of glycoproteins are attached to the polypeptide chains through covalent glycosidic bonds , either O - glycosidic bonds of oligoglycosyls to alcoholic groups in the side chains of residues of serine , threonine or hydroxylysine , or N - glycosidic bonds of oligoglycosyls to the amide groups in the side chains of asparagine . Ser Asn
In certain glycoprotein types, more than two or three glycosyls can be attached through glycosidic bonds to one glycosyl so that branching occurs in those saccharide components. Because the saccharide component of glycoproteins may comprise many different glycosyl types , there is a very high degree of diversity in the structures of both protein and saccharide component of glycoproteins.
– increase in the polarity (and solubility) of a protein,
– have their share of the surface electric charge (if alduronic acids, sialic acids, and sulfate esters are among the components)
– prevent hydrolysis of the protein by proteinases,
– may control the biological half-life (desialylation of circulating glycoproteins),
– take part in the right orientation of the protein in membranes and stabilizes the correct (functional) conformation of the protein,
– may direct the transport across the membrane,
– mostly are prerequisites for specific binding of hormones and other signalling molecules to the cellular membrane receptors ,
– provide recognition of cells ( antigenic determinants on the outer surface of cells),
– are important for binding of viruses or other microorganisms onto the cells,
Functions of saccharide components of glycoprotein s – examples:
The major glycoprotein types – Blood plasma glycoprotein type most of the blood plasma proteins (not plasma albumin!) and many integral membrane glycoproteins that form glycocalyx – Mucus glycoprotein type (mucins) secreted by epithelial cells with protective and lubricative functions (among others, also glycoproteins with blood AB0 group determining structures) – Proteoglycans – produced by fibroblasts – Collagen type – products of fibroblasts The major collagen types are glycosylated only very poorly (about 1 % of molecular weight)
Blood plasma glycoprotein type Oligosaccharides are attached to the amide group of asparaginyl residues through N -glycosidic bonds: Asn
The boxed area encloses the pentasaccharide core common to all N -linked glycoproteins. Bi-antennary type (complex type) High-mannose type is a common precursor in biosynthesis of other types
Integral membrane penetrating ( glyco ) proteins Type I Type II less common "reversed" type, e.g. transferrin receptor Type III PI-link Type IV e.g. superfamily of receptors interacting with G-proteins
The molecules can comprise up to 75 % saccharides forming so very viscous solutions (a typical feature of secretion from secretory cells of mucosa. Mucus glycoprotein type Antifreeze glycoproteins in antarctic fish prevents from freezing (inhibits ice nucleation) – galactosyl–N-acetyldeoxyaminogalactosyl serine / threonine Saccharide component is attached through O– glycosidic bond to the alcoholic groups of seryl or threonyl residues:
In the membranes or red blood cells, those antigenic determinants occur as glycolipids (attached to membrane ceramide). In some humans ( "secretors"), AB0 antigens appear as the saccharide component of glycoproteins secreted by epithelial cells.
Proteoglycans If not thinking of dense collagen connective tissue and bone, proteoglycans represent the most voluminous component of amorphous ground substance in connective tissue, which fill in the space among fibres and cells. In proteoglycans, numerous (very approximately 100) chains of different glycosaminoglycans (that include 10 –100 monosaccharide units) bind through glycosidic bonds the core protein forming so aggregates called monomeric proteoglycans or agrecans . The most typical link is the link of the innermost sequence of glycosaminoglycans – galactosyl–galactosyl– xylyl serine
A large number of simple monomeric proteoglycans (agrecans) bind their globular domains of core proteins non-covalently to a long chain of hyaluronic acid. Huge aggregates are formed in this way namely in hyaline cartilages. They contribute to the resistance of a cartilage to mechanical pressure and to its elasticity . Proteoglycans are highly hydrated , and numerous carboxylate and sulfate groups bind due to negative electric charges large amounts of hydrated cations. hyaluronate Monomeric proteoglycan (agrecan) agrecans In spite of its large size, core protein of proteoglycans represents only about 5 – 15 % mass of the proteoglycan. The agrecan structure resembles a bottle-brush. The bristles are N -linked oligosaccharides, O -linked oligosaccharides or keratan sulfates, O -linked Xyl-Gal-Gal-chondroitin sulfates.
Collagen type glycoproteins In collagen types I and III, small number of galactosyls or glucosyl-galactosyls are attached to 5- hydroxylysyl residues through O -glycosidic bonds : α-D-galactosyl hydroxylysine ( 2- O- -D-glucosyl )
Some structural divergences in collagen types I , II , III , and IV Collagen I is the most common type ( skin , bones , tendons, dentin), resisting to tensile strength. Slightly glycosylated ( < 1 % saccharides), no cysteinyl residues. Collagen II is the major type present in the hyaline cartilage of joints. High degree of glycosylation , no cysteinyl residues. Collagen III (skin, aorta, uterus) is an elastic type in the form of thin reticuline fibrils . Very low glycosylation, cysteinyl residues are present, small number of disulfide bridges. Collagen IV is the typical type of basement membranes (among others renal glomeruli, capsule of the eye lens) forming the non-fibrillar network that stabilizes a thin membrane. Its flexible triple helices include some non-helical segments and at their C-ends there are globular domains . Saccharidic component about 15 %, cysteinyl residues and disulfide bridges are present.
Haemoproteins are proteins with different functions containing covalently bound haem . (In American English, haem is spelled "heme".) Important haemoproteins: Haemoglobin transporting dioxygen (and its derivatives), myoglobin binding dioxygen within skeletal muscles, cytochromes of different types that transport electrons in the terminal respiratory chain (or similar electron transferring systems, e.g. cytochromes P 450), some enzymes catalyzing oxidations-reductions, for example catalase and peroxidase . Haem is an Fe-containing prosthetic group. The heterocyclic ring system of haem is a porphyrin derivative . It consists of four pyrrole rings linked by four methene bridges, with a centrally bound Fe(II) atom.
Porphin is a planar , fully conjugated system. N N N N H H Porphin Cyclic tetrapyrroles N N N N H H Pyrrole rings A B C D N N N N H H Methene bridges α N N N N H H Numbering of positions of substituents 2 3 (1) (20) 7 8 12 13 17 18
N HN N NH CH 3 COOH COOH CH 3 CH 3 CH 3 CH 2 CH 2 protoporphyrin IX Derivatives of porphin (by substitution) are porphyrins – intensively coloured, there is a fully conjugated system of double bonds, or porphyrinogens , in which some of the bridges are methylene bridges (much less absorption of visible light). Kinds of substituents: Two sorts – 4 remainders of acetic acid, 4 remainders of propionic acid ( uroporhyrinogens/uroporphyrins ), or – 4 methyl groups and 4 remainders of propionic acid ( coproporphyrinogens/coproporphyrins ); Three sorts – 4 methyl groups, 2 vinyl groups and 2 remainders of propionic acid ( protoporphyrins ).
The side chains that fill in the interhelical space are not drawn. The tertiary structure of haemoglobin subunit
Cytochromes are haem-containing proteins , which are one-electron carriers due to reversible oxidation of the iron atom: Mammalian cytochromes are of three types, called a , b , and c . All these types of cytochromes occur in the mitochondrial respiratory (electron transport) chain. Cytochromes type b (including cytochromes class P 450 ) occur also in membranes of endoplasmic reticulum and elsewhere. N N N N F e 2+ N N N N F e 3+ + e – – e –
Some differences in cytochrome structures Cytochrome c M r 12 000, the central Fe ion is attached by coordination to N-atom of His 18 and to S-atom of Met 80 ; two vinyl groups bind covalently S-atoms of Cys 14 and Cys 17 . The haem is dived deeply in the protein terciary structure so that it is unable to bind dioxygen, carbon monoxide or CN – ion. Cyt c is water-soluble, peripheral protein that moves on the outer side of the inner mitochondrial membrane. Cytochrome aa 3 M r ~ 170 000, the central Fe ion is attached by coordination to two histidyl residues; one of substituents is a hydrophobic isoprenoid chain, another one is oxidized to formyl group. The haem a is the accepts an electrons from the copper centre A (two atoms Cu A ). Its function is inhibited by carbon monoxide, CN – , HS – , and N 3 – anions. Haem a of cytochrome aa 3 Haem of cytochrome c
bilanes (with substituents at β-positions) CH 3 H H H O O N N N N H CH 3 CH 3 CH 3 CH 2 CH 2 HOOC COOH bilirubin IX α H H H O O N N N N H Linear tetrapyrroles The product of oxidative splitting of haem is green biliverdin, Fe 3+ ion and carbon monoxide.