Learn about Proteoglycans and Peptidoglycans Today

Proteoglycans and Peptidoglycans:

            Connective tissues are composed of insoluble protein fibers embedded in a matrix of proteoglycans. The connective tissues combine tissues together and provide strength and support for the organs and other structures of the body. Their properties depend on the proportion of different components present. A tissue of very high tractile strength, the Achilles tendon, is comprised of about 32% collagen and 2.6% elastin, whereas an elastic tissue, the ligamentum nuchae, is composed of about 32% elastin and 7% collagen. The proteins and proteoglycans are synthesized by connective tissue cells: fibroblasts, chondroblasts (cartilage), and osteoblasts (bone). Connective tissue also contains blood and lymphs vessels and various transient cells including macrophages and mast cells. Adipose tissue is a specialized form of connective tissue consisting of a collection of adipocytes that cluster between the protein fibers.
Collagen:

• Collagens are extracellular proteins of connective tissue and they make up about one third of all body protein. 
• They are a family of related glycoproteins in which hydroxylysyl molecules provide the sites for attachment of glucose, galactose, or an α(1 à 2) glucosylgalactose residue through a β-O-glycosidic linkage. 
• In brief, the production of collagen can be considered to happen in two stages: intracellular and extracellular. The intracellular stage consists of the formation of procollagen from precursor polypeptide chains that undergo, in sequence, hydroxylation, glycosylation, occurance of a triple helix, and secretion. 
• The extracellular stage consists of the conversion of procollagen to tropocollagen by limited proteolysis from the amino and carboxyl termini, self-assembly of tropocollagen residues into fibrils, and finally cross-linking of the fibrils to form collagen fibers. 
• Collagen exists mostly as fibrous protein; however, in the basement membrane of many tissues, including kidney glomeruli and the lens capsule, it is present in a nonfibrous form. 
• The unique property of each connective tissue depends on the composition and organization of collagen and other matrix components.

Collagen Types:

• More than 16 various types of collagen have been noticed. They constitute the most abundant family of proteins in the human body. 
• The collagens are encoded by 28 genes dispersed in at least 12 different chromosomes. The individual genes are identified by both collagen type and the polypeptide chain. For example, COL1A2 gene codes for type I collagen (COL l) and α2 (A2) polypeptide chain. 
• The tissue distribution of collagens also varies. Type I collagen consists of two identical chains of α1(I) and one chain of α2; it is the major connective tissue protein of skin, bone, tendon, dentin, and some other tissues. 
• Type II collagen comprises of three identical chains of α1(II); it is found in cartilage, cornea, vitreous humor, and neural retinal tissue. 
• Type III collagen consists of three identical chains of α1(III); it is present, along with type I collagen, in skin, arteries, and uterine tissue. 
• Type IV collagens are composed of α1(IV) to α5(IV) and are found in the basement membranes of various tissues. 
• Collagen types I, II, III, V, and XI which are fibrillar, differs with type IV collagen which is not fibrillar in structure.
• The synthesis of collagens in cultured cells has aided the understanding of collagen biochemistry. The tissue specificity of various types of collagen is also appeared in cultured cells obtained from appropriate tissues. 
• For example, human fibroblasts and smooth muscle cells contain both types I and III collagens; epithelial and endothelial cells form type IV collagen; and chondroblasts synthesize type II collagen.

Turnover of Collagen and Tissue Repair:

           The degradation of collagen in the connective tissue matrix is performed by enzymes known as collagenases. Part of the degradation may also involve neutral proteinases. A metalloenzyme explicit for collagen catalyzes the cleavage of the triple helix at a single peptide bond, situated at a distance from its N terminus related to about three fourths the length of the tropocollagen molecule. The cleavage sites in type I collagen are the peptide bonds of Gly-Ile of the α1-chain and of Gly-Leu of the α2-chain. The role and regulation of collagenase activity in vivo are not clearly known. The activity of the enzyme might be regulated through the formation of an enzyme-inhibitor complex or by the activation of a proenzyme, or both. A substantial amount (20-40%) of newly synthesized polypeptide chains of collagen undergoes intracellular degradation. This degradation, which appears to happen in lysosomes, might be essental in regulating the amount of collagen synthesized and in removing any defective or abnormal polypeptide chains that may be synthesized. Turnover of collagen in humans has been estimated by measurement of urinary hydroxyproline, which occurs mostly in the form of a peptide. Hydroxyproline makes up about 9-13 % of collagen residues and is not reutilized. Two aspects of hydroxyproline metabolism affect its use in the assessment of the true rate of collagen turnover:
1. Hydroxyproline is rapidly metabolized in a pathway initiated by the enzyme hydroxyproline oxidase.
2. Hydroxyproline is present in C1q, which is synthesized at a rate estimated to be about 4.5 mg/kg
per day.
                The first perspective has been overcome by estimation of urinary hydroxyproline in people with an inherited deficiency of hydroxyproline oxidase. Such persons excrete 0.3 g of hydroxyproline per day in urine, which corresponds to about 2.25 g of hydroxyproline-containing proteins, most of which is collagen. Total protein catabolism in a normal well-fed adult is about 300g; thus, collagen catabolism constitutes only about 0.8% of total protein catabolism. Collagen turnover has important clinical implications. The location, amount, type, and form of collagen rely on the organized control of its synthesis and degradation. In tissue damage (physical, chemical, infectious, or radiation), repair process consists regeneration and fibrous connective tissue formation. Regeneration is the most appreciable form of repair: a cut surface of an epidermis is replaced with new epidermis; scattered dead liver cells are replaced with new liver cells. Degradation products of type I collagen, namely N-telopeptides, C-telopeptides, hydroxyproline, and the collagen cross-links pyridinolone and deoxypyridinolone, have been used as markers of osteoclast activity in bone resorption. 

               Collagen metabolite markers used for bone formation reflecting osteoblast activity are procollagen type I carboxyterminal propeptide and procollagen type I N-terminal propeptide. Both of these propeptides are cleaved byproducts generated from the conversion of procollagen to tropocollagen. Osteocalcin and bone-specific alkaline phosphatase are also used as bone formation markers. Biochemical markers of bone turnover are used for assessing and monitoring therapy in a number of diseases of bone including osteoporosis. The cells of the body fall into three groups according to regenerative capacity: labile, stable, or permanent. 

                 The labile cells multiply throughout life and are maintained at an optimal level by continual proliferation of reserve cells. Labile cells are occured in all epithelial surfaces, the spleen, and lymphoid and hematopoietic tissues. Stable cells have the potential to regenerate but do not normally undergo replication. However, under appropriate stimuli, they can proliferate rapidly. Stable cells include parenchymal cells of all glandular organs of the body and mesenchymal cells (e.g., fibroblasts, smooth muscle cells, osteoblasts, chondroblasts, and vascular endothelial cells). Permanent cells do not undergo significant replication postnatally. Nerve cells, skeletal muscle, and cardiac muscle cells are permanent cells.

Elastin:

• Elastin is a fibrous, insoluble protein that is not a glycoprotein however is available with collagen in the connective tissues. 
• Connective tissues rich in elastic fibers show a characteristic yellow color. Elastic fibers are highly branched structures responsible for physiological flexibility. 
• They are capable of extending in two dimensions and are found most prominently in tissues subjected to continual high-pressure differentials, tension, or physical deformation. 
• Elastin gives to these tissues the properties of stretchability and subsequent recoil that depend only on the utilization of some physical force. Tissues rich in elastic fibers include the aorta and other vascular connective tissues, various ligaments, and the lungs. 
• Microscopically, elastic fibers are more slender than collagen fibers and lack longitudinal striations. Elastin fibers can be seggregated into amorphous and fibrillar components. 
• The amorphous component comprises of elastin, which is characterized by having 95% nonpolar amino acids and two unique lysine-derived amino acid residues, desmosine and isodesmosine.
• Mature elastin is a linear polypeptide, tropoelastin, which possess a molecular weight of about 72,000 and contains about 850 amino acid residues. Although glycine accounts for one third of the residues, the repeat sequence GlyX-Y characteristic of collagen is not present in elastin. 
• Instead, glycine residues are present in the repeat units Gly-Gly-Val-Pro, Pro-Gly-Val-Gly-Val, and Pro-Gly-Val-Gly-Val-Ala. Elastin is generally rich in the nonpolar amino acids like alanine, valine, and proline. 
• Elastin possesses no hydroxylysine or sugar molecules. A feature of mature elastin is the presence of covalent cross-links that link elastin polypeptide chains into a fiber network. 
• The major cross-linkages involve desmosine and isodesmosine, both of which are derived from lysine amino acid. Several regions which are rich in lysine residues can provide cross-links. 
• Two such regions that contain peptide sequences that are occurred many times in tropoelastin have the primary structure-Lys-Ala-Ala-Ala-Lys- and - Lys-Ala-Ala-Lys-. 
• The aggregation of lysine residues with alanine residues provides the appropriate geometry for the occurance of cross-links. Cross-linking elastin occurs extracellularly. 
• The polypeptides are formed intracellularly by connective tissue cells, according to the same principles for the synthesis of other export proteins. 
• After transport into the extracellular space, the polypeptides experience crosslinking. The critical step is the oxidative deamination of certain lysyl molecules, catalyzed by lysyl oxidase enzyme. 
• These lysyl residues are converted to very reactive aldehydes known as allysine residues. Through an unknown process, three allysine residues and one unmodified lysine residue react to synthesize a pyridinium ring, alkylated in four positions, which is the basis for the cross-links. 
• The cross-links might occur within the same polypeptide or between two to four different polypeptide chains. Present models of elastin structure explains that only two polypeptide chains are enough to form the desmosine cross-link, one chain giving a lysine and an allysine residue, the other contributing two allysine molecules. 
• Elastin polypeptides are also cross-linked by condensation of a lysine residue with an allysine molecule, followed by reduction of the aldimine to give a lysinonorleucine residue. 
• These linkages occur to a much lesser extent than in collagen. Occurance of cross-linkages in elastin can be prevented by inhibition of lysyl oxidase.

Turnover of Elastin:

• The turnover rate of mature elastin in healthy persons is relatively low. Insoluble elastin in healthy elastic tissue is usually stable and subjected to minimal proteolytic degradation. 
• In several clinical conditions, increased degradation of fragmentation of elastic fibers may play a significant role. The interaction between insoluble elastin and soluble elastolytic enzymes, and the regulation of these enzymes, may shed light on certain cardiovascular diseases, in view of the role of elastin in arterial dynamics. 
• Elastolytic proteinases (elastases) are found in pancreatic tissue, polymorphonuclear leukocytes, macrophages, and platelets. These enzymes exhibit broad specificity. 
• They catalyze preferential cleavage of peptide bonds adjacent to aliphatic amino acids, namely, glycine, alanine, and valine, which are present in high amounts in elastin. Elastases degrade elastin at neutral or slightly alkaline pH. 
• The pancreas secretes elastase as an inactive precursor (zymogen) known as proelastase, which is converted to its active form by trypsin in the duodenum; elastase takes part in the digestion of dietary proteins. 
• This enzyme has a structure homologous to that of other pancreatic serine proteinases, including the amino acid sequence at the active site. Polymorphonuclear leukocyte elastase is also a serine proteinase. 
• Elastases are inactivated by serum α1-proteinase inhibitor and α2-macroglobulin; thus, their proteolytic action may be checked to prevent indiscrimination digestion of elastin-containing tissues. An enzyme that degrades only soluble forms of elastin and exhibits trypsin-link activity has been described. Its biological significance is unknown.

Proteoglycans:

Proteoglycans are high molecular-weight, complex molecules with different structures and functions. They are polyanionic substances containing a core protein to which at least one glycosaminoglycan chain is covalently attached. Proteoglycans are major compounds of connective tissue and participate with other structural protein constituents, namely, collagen and elastin, in the organization of the extracellular matrix.
Types, structure and function of Glycosaminoglycans:

• Six classes of glycosaminoglycans have been described. All are heteropolysaccharides and contain repeating disaccharide units. In five glycosaminoglycans, the disaccharide units consist of amino sugars alternating with uronic acids. 
• In keratan sulfate, the uronic acid is substituted by galactose. The amino sugars are generally occur as N-acetyl derivatives. In heparin and heparan sulfate, however, most amino sugars are found as N-sulfates in sulfamide link-age, with a small number of glucosamine residues as N-acetyl derivatives.
• With the exception of hyaluronate, all glycosaminoglycans possess sulfate groups in ester linkages with the hydroxyl groups of the amino sugar molecules. 
• The vitreous space behind the lens of the eye is filled with a thick, gelatinous solution of hyaluronate free of fibrous proteins and transparent to light. 
• Hyaluronate is not sulfated, and there is no evidence that it is linked to a protein molecule, as are the other glycosaminoglycans. 
• Hyaluronic acid is also present in synovial fluid in joint cavities that join long bones, bursae, and tendon sheaths. The viscous and elastic properties of hyaluronic acid grant to the functioning of synovial fluid as a lubricant and shock absorber. 
• Hyaluronate is depolymerized by hyaluronidase, which cleaves the β (1à 4) glycosidic linkages between N-acetylglucosamine and glucuronate. 
• Some pathogenic microbes secrete hyaluronidase, which breaks down the protective barrier of hyaluronate and renders the tissue progressively susceptible to infection. 
• Hyaluronidase, found in spermatozoa, might facilitate fertilization by cleaving the outer mucopolysaccharide layer of the ovum, thereby enabling the sperm to penetrate it. 
• Hyaluronidase is used therapeutically to enhance dispersion of drugs administrated in various parts of the body.
• Heparin is a heterogeneous glycosaminoglycan found in tissues that contain mast cells (e.g., lungs and perivascular connective tissue). 
• Its primary physiological function is unknown, but it is a powerful inhibitor of blood clotting and is used therapeutically for that purpose. 
• The therapeutic anticoagulant action of heparin is due to its ability to produce conformational changes in the proteinase inhibitor antithrombin. Both activated factor X (factor Xa) and thrombin are inactivated by a heparinantithrombin complex. 
• The antithrombin is homologous in structure with the α1-antitrypsin family of proteinase inhibitors and is a suicide substrate for factor Xa and thrombin. The proteinase binds to a specific Arg-Ser peptide bond present at the reactive site of antithrombin. 
• This 1:1 antithrombin-thrombin is a stable inactive complex. The function of heparin is catalytic. After the formation of the heparin-mediated antithrombinthrombin complex, heparin is released to initiate another cycle. 
• Factor Xa is inhibited by a specific heparin pentasaccharide bound to antithrombin. However, the inhibition of thrombin requires an antithrombin bound to the heparin consisting of the specific pentasaccharide that consists of chains of at least 18 monosaccharide units. 
• Tendons have a high content of collagen and sulfated glycosaminoglycans (chondroitin and dermatan sulfates). Tendons are fibrous cords that fuse with skeletal muscle at each end and penetrate bones at the two sides of a joint. Thus, tendons are aligned along their long axis, providing flexible strength in the direction of the muscle pull.

Hyaluronic acid

Chondroitin sulfate

Dermatan sulfate

Keratan sulfate 

Heparan sulfate

Heparin

 Glycosaminoglycans

Glycosaminoglycan	Amino Sugar	Uronic Acid	Type of Sulphate Linkage	Tissue Distribution
Hyaluronate	D-Glucosamine	D-Glucuronate	None	Connective tissues, cartilage, synovial fluid, vitreous humor, umbilical cord
Chondroitin sulfate	D-Galactosamine	D-Glucuronate	4-0- and/or 6-0-sulfate on galactosamine	Cartilage, bone, skin, cornea, blood vessel walls
Dermatan sulfate	D-Galactosamine	L-Iduronate, D-Glucuronate	4-0-sulfate on galactosamine; 2-0-sulfate on iduronate	Skin, heart valve, tendon, blood vessel walls
Keratan sulfate	D-Glucosamine	None	6-0-sulfate on both carbohydrate residues	Cartilage, cornea, intervertebral disks
Heparin sulfate	D-Glucosamine	D-Glucuronate, L-Iduronate	6-0-sfate and N-sulfate on glucosamine	Lung, blood vessel walls
Heparin	D-Glucosamine	L-Iduronate, D-Glucuronate	2-0-sulfate on iduronate; 6-0-sulfate and N-sulfate on glucosamine	Lung, liver, skin,intestinal mucosa

Peptidoglycans:

• Peptidoglycans are compunds of bacterial cell walls and comprises of heteropolysaccharide chains cross-linked by short peptide chains. 
• These cell walls bear the antigenic determinants; when exposed to them, humans develop specific antibodies to defend against bacteria. 
• Bacterial virulence also sometimes depends on substances associated with the cell wall. Cell wall synthesis is the target for the antibiotics like penicillins and cephalosporins. 
• Bacterial cell walls are rigid and complex, enable the cells to withstand severe osmotic shock, and survive in a hypotonic environment. The contents of a bacterium can exert an osmotic pressure as high as 20 atm. 
• At cell division, the walls rupture and reseal rapidly. Bacteria are classified into two groups on the basis of a staining reaction discovered by Gram in 1884. 
• In this reaction, the cells do or do not retain a crystal violet iodine dye complex after an alcohol wash. Cells that retain the stain are gram-positive; those that do not are gram negative. 
• This empirical classification divides bacteria into two classes that differ in cell wall structure. Gram-positive bacteria are surrounded by a cytoplasmic membrane with a bilayer structure similar to that of eukaryotes and a thick, bag shaped cell wall. 
• The cell wall, about 25 nm wide, consists of peptidoglycans and polyol phosphate polymers known as teichoic acids. Gram-negative bacteria do not contain teichoic acids but have an outer membrane system external to the plasma membrane and the peptidoglycan. 
• The constituents of this second outer membrane system are phospholipids, lipopolysaccharides, and proteins. The peptidoglycan layer is connected to the outer layer via lipoprotein. 
• The region between the inner membrane and the peptidoglycan layer is known as the periplasmic space. This space contains enzymes and other proteins that digest impermeable nutrients and transport low-molecular-weight nutrients, such as amino acids, sugars, and specific ions. Lipopolysaccharides are complex amphipathic molecules located on the outer leaflet of the outer membrane. 
• Their structure in Enterobacteriaceae comprises three regions: a phospholipid (A), a phosphorylated core oligosaccharide, and a polysaccharide chain known as the 0 side chain or 0 antigen. The core oligosaccharide and the 0 side chain regions contain several unusual sugars and form the hydrophilic portion of the molecule that projects outward.
• Lipopolysaccharides serve as a barrier against invading organisms, are highly toxic and are also known as endotoxins. The fever that occurs in gram-negative bacterial infections is caused by endotoxins. The outer membrane also contains a protein known as porin. 
• The trimeric forms of porin form channels for the passage of small ionic molecules through the outer membrane. Other transport systems exist for higher molecular weight ionic substances. Both gram-negative and gram-positive bacteria contain peptidoglycan as the main structural component of their cell walls. However, gram-negative bacteria have only a single layer of peptidoglycan, whereas gram-positive cells have several layers with cross-linkages between them. 
• The polymeric structure of peptidoglycan surrounds the entire inner membrane and the cytoplasm of the cell. Consisting of a glycan composed of amino sugars in one dimension, a cross-linked peptide moiety in the second dimension, and an interpeptide bridge in the third dimension, peptidoglycan projects in different planes. 
• This three dimensional, covalently bound network offers considerable resistance to outward pressure and confers mechanical stability on the cell wall. The glycan is called murein and is a polymer of alternating units of Nacetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). 
• The sugar residues are linked by β(1 à 4) glycosidic bonds. N-Acetylmuramic acid is the 3-O-D-lactic acid ether of N-acetylglucosamine, the carboxyl group of its lactic acid side chain being condensed with the cross-linking peptide. 
• This peptide usually consists of four amino acid residues that are alternately L- or D-amino acids. The bacterial cell wall is one of the few structures containing a significant amount of D-amino acid residues. 
• In S. aureus,the tetrapeptide sequence is L-alanyl-D-g-isoglutaminylL-lysyl-D-alanine. The interpeptide chain is a pentaglycine chain that connects the terminal D-alanyl residue of one tetrapeptide to an L-lysyl residue in the third amino acid position in another tetrapeptide. 
• The chemical linkages are as follows: the amino-terminal group of pentaglycine is joined with the carboxyl group of D-alanine, and the carboxyl terminal of pentaglycine is linked with the e-NH2 of L-lysine. Both of the sequence variations of the tetrapeptide and the cross-linking pattern are characteristic of the shape and species of the bacterium. 
• In some bacteria, in addition to the cell wall, substances form capsules or slime layers external to the
• cell wall. These layers are not essential for growth and multiplication but may be important for survival of the organism in harsh environments, e.g., in preventing desiccation and serving as a barrier against phage attack. 
• The capsular layer also provides a charged surface. Streptococcus mutans, which plays a significant role in dental caries and plaque formation, produces an extracellular 1, 3-glucan that enables the bacteria to adhere to teeth. 
• In the formation of dental caries, possibly the most widespread human pathological process, dietary carbohydrates promote bacterial growth and produce noxious metabolites. Penicillins prevent cell wall synthesis in susceptible organisms by inhibiting a late step in the enzymatic synthesis of peptidoglycan.

Peptidoglycan

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