GlycoproteinsHaemoproteins

Medical Chemistry

Lecture 15 2007 (J.S.)

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Glycoproteinsare glycosylated proteins, i.e. proteins that comprise a saccharidecomponent attached to amino acid side chains by glycosidic bond.

In most glycoproteins, the size of the saccharide components is inthe 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.

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)

Saccharide components of glycoproteins may comprise

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Saccharide components of glycoproteins are attached to thepolypeptide chains through covalent glycosidic bonds,

either O-glycosidic bonds of oligoglycosyls toalcoholic groups in the side chains of residues of serine, threonine or hydroxylysine,

or N-glycosidic bonds of oligoglycosyls to the amide groupsin the side chains of asparagine.

Ser Asn

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In certain glycoprotein types, more than two or three glycosylscan be attached through glycosidic bonds to one glycosyl sothat branching occurs in those saccharide components.

Because the saccharide component of glycoproteinsmay comprise many different glycosyl types, there is a veryhigh degree of diversity in the structures of both protein andsaccharide component of glycoproteins.

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– 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 glycoproteins– examples:

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The major glycoprotein types

– Blood plasma glycoprotein typemost of the blood plasma proteins (not plasma albumin!)and many integral membrane glycoproteins that form glycocalyx

– Mucus glycoprotein type (mucins) secreted by epithelial cellswith 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)

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Blood plasma glycoprotein type

Oligosaccharides are attachedto the amide group of asparaginyl residuesthrough N-glycosidic bonds:

Asn

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The boxed area enclosesthe pentasaccharide corecommon to all N-linkedglycoproteins.

Bi-antennary type (complex type)High-mannose typeis a common precursorin biosynthesis of other types

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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

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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 bondto the alcoholic groups of seryl or threonyl residues:

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In the membranes or red blood cells, those antigenic determinantsoccur as glycolipids (attached to membrane ceramide).In some humans ("secretors"), AB0 antigens appear as the saccharide component of glycoproteins secreted by epithelial cells.

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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 ofglycosaminoglycans

–galactosyl–galactosyl–xylyl serine

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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.

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Collagen type glycoproteins

In collagen types I and III, small number of galactosyls orglucosyl-galactosyls are attached to 5-hydroxylysyl residuesthrough O-glycosidic bonds:

α-D-galactosyl hydroxylysine( 2-O--D-glucosyl )

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Some structural divergences in collagen types I, II, III, and IV

Collagen I is the most commontype (skin, bones, tendons, dentin), resisting to tensile strength. Slightly glycosylated (< 1 % saccharides),no cysteinyl residues.

Collagen II is the major type presentin the hyaline cartilage of joints.High degree of glycosylation,no cysteinyl residues.

Collagen III (skin, aorta, uterus)is an elastic type in the form ofthin 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.

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Haemoproteins

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Haemoproteinsare proteins with different functions containingcovalently 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.

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Haem

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N N

NN H

H

Porphin

Cyclic tetrapyrroles

N N

NNH

H

Pyrrole rings

A B

CDN N

NNH

H

Methene bridges

α

N N

NNH

H

Numbering of positions of substituents

2

3

(1)

(20)

7

8

12

1317

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Porphin is a planar,fully conjugated

system.

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N HN

NNH

CH3

COOHCOOH

CH3 CH3

CH3

CH2

CH2

protoporphyrin IX

Derivatives of porphin (by substitution) are

porphyrins – intensively coloured, there is a fully conjugated system

of double bonds, orporphyrinogens, 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).

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The side chains that fill in the interhelical space are not drawn.

The tertiary structure of haemoglobin subunit

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Cytochromesare haem-containing proteins, which are one-electron carriers due to reversible oxidation of the iron atom:

N N

NN

Fe 2+

N N

NN

Fe 3+

+ e–

– e–

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 P450) occur also in membranes of endoplasmic reticulum and elsewhere.

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Haem a of cytochrome aa3

Haem of cytochrome c

Some differences in cytochrome structures

Cytochrome c

Mr 12 000, the central Fe ion is attached by coordination

to N-atom of His18 and to S-atom of Met80;two vinyl groups bind covalently S-atoms of Cys14 and Cys17.

The haem is dived deeply in the protein terciarystructure so that it is unable to bind dioxygen,carbon monoxide or CN– ion.

Cyt c is water-soluble, peripheral protein that moveson the outer side of the inner mitochondrial membrane.

Cytochrome aa3Mr ~ 170 000, the central Fe ion is attached

by coordination to two histidyl residues;one of substituents is a hydrophobic isoprenoidchain, another one is oxidized to formyl group.

The haem a is the accepts an electrons fromthe copper centre A (two atoms CuA).

Its function is inhibited by carbon monoxide,CN–, HS–, and N3

– anions.

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bilanes (with substituents at β-positions)

CH3

H HHOO NN NN

H

CH3CH3 CH3

CH2 CH2

HOOC COOH

bilirubin IXα

H HHOO NN NN

H

Linear tetrapyrroles

The product of oxidative splitting of haemis green biliverdin, Fe3+ ion and carbon monoxide.