Mucopolysaccharides, Sterols and Sphingolipids

• Particular metabolism encompasses Mucopolysaccharides, sterols, steroids, and sphingolipids.

Glycosaminoglycans (Mucopolysaccharides)

• Serve as information carriers and mediators in cell interactions, acting as labels for proteins.
• Specific molecules containing carbohydrates participate in cell recognition, adhesion, migration, blood clotting, immune response, and wound healing.
• Glycoconjugates form through covalent joining of carbohydrates to proteins or lipids.
• Complex carbohydrates containing amino sugars and uronic acids may be attached to a protein to form a proteoglycan.
• Proteoglycans act as connective tissue's ground substance, and occupy space for cushioning/lubrication.
• Hyaluronic acid, chondroitin sulfate, and heparin are examples of mucopolysaccharides.

Proteoglycans

• Proteins containing covalently linked glycosaminoglycans (GAGs).
• At least 30 have been characterized, including syndecan, betaglycan, serglycin, perlecan, aggrecan, versican, decorin, biglycan, and fibromodulin.
• Proteins covalently bound to GAGs are core proteins that vary in tissue distribution, core protein nature, attached GAGs, and function.
• Uses recombinant DNA technology to get information about their structure.
• Amount of carbohydrate is up to 95% of the weight of a proteoglycan.
• Link proteins interact noncovalently with core protein molecules for GAG chain projection.
• Hyaluronic acid, chondroitin sulfate, keratan sulfates I & II, heparin, heparan sulfate, and dermatan sulfate are seven GAGs.

Mucopolysaccharides

• GAGs are unbranched polysaccharides made of repeating disaccharides containing D-glucosamine, or D-galactosamine.
• The other component of the repeating disaccharide (except in keratan sulfate) is L-glucuronic acid (GlcUA) or L-iduronic acid (IdUA).
• They are found in every tissue, particularly in the extracellular matrix, associated with components like collagen and elastin.
• Some proteoglycans bind to collagen and others to elastin, determining matrix structure, also bind TGF-β, fibronectin, and laminin.
• The GAGs present in the proteoglycans are polyanions that attract cations into the extracellular space, contributing to tissue turgor.
• At low concetrations, GAGs form a gel, acting as sieves that restrict large molecules while allowing small molecules to diffuse freely.

GAG Properties

• GAGs vary in amino sugar composition, uronic acid composition, linkages, chain length, presence/absence of sulfate groups, nature of core proteins, core protein linkages, tissue, subcellular distribution, and biologic functions.
• Three typical linkages of GAG and core protein:
• O-glycosidic linkage: Formed between N-acetyl galactosamine (GalNAc) and serine or threonine; typically in keratan sulphate II.
• N-glycosylamine linkage: Formed between N-acetyl glucosamine (GlcNAc) and amide N of asparagine; and N-linked glycoproteins.
• O-glycosidic linkage: Formed between xylose (Xyl) and serine; unique to proteoglycans.

Hyaluronic Acid

• This GAG is an unbranched chain of repeating GlcUA and GlcNAc units.
• Exists in bacteria, virtually all animal tissues, and primarily in hydrated tissues, in embryonic tissues.
• It supports cell migration during morphogenesis and wound repair.
• Ability to attract water helps loosen the ECM.
• With chondroitin sulfates present in cartilage, contributes to its compressibility.

Chondroitin Sulfates

• Linked to proteoglycans via the Xyl-Ser O-glycosidic bond, prevalent in cartilage components.
• Repeating disaccharide containing GlcUA similar to hyaluronic acid, but with GalNAc replacing GlcNAc.
• The GalNAc is sulfated at the 4' or 6' position.
• Play an important role in ECM structure.
• Found at calcification sites in endochondral bone, also a major component of cartilage.
• Thought to be signaling molecules that prevent repair of nerve endings after injury.

Keratan Sulfates I & II

• Consists of repeating Gal-GlcNAc disaccharide units, with sulfate attached to the 6' position of GlcNAc or galactose.
• Keratan sulfate I was originally isolated from the cornea.
• Keratan sulfate II came from cartilage.
• They vary in structural links because tissue distribution isn't specific, the classification depends on the different protein linkage.
• In the eye, they lie between collagen fibrils and are critical for corneal transparency.
• Changes in proteoglycan composition disappear as the cornea heals.

Heparin

• The repeating disaccharide contains glucosamine (GlcN) and either of the two uronic acids.
• A majority of the GlcN residues have N-sulfated amino groups, while a few are acetylated; the GlcN also has a sulfate attached to carbon 6.
• Heparin proteoglycan's protein molecule has serine and glycine residues.
• About two thirds of serine residues have GAG chains, generally 5-15 kDa, but sometimes larger.
• Heparin is in mast cell granules and in the liver, lung, and skin.
• Important anticoagulant that binds with factors IX and XI, but interacts with plasma antithrombin.
• It binds to lipoprotein lipase in capillary walls, releasing the enzyme into circulation.

Heparan Sulfate

• Exists on cell surfaces as a proteoglycan, and it is extracellular.
• A glucosamine is contained with fewer N-sulfates versus heparin and GlcUA is it's predominant uronic acid.
• Core proteins of the membranes of cells span the membrane, act as receptors, and mediate cell growth/communication.
• It plays a major role in glomerular filtration, also found in the kidney's basement membrane with type IV collagen & laminin.

Dermatan Sulfate

• It is in wide distribution in animal tissues, having a structure similar to chondroitin sulfate, except contains IdUA in an α-1,3 linkage to GalNAc.
• As in heparin and heparan sulfate, the formation of IdUA occurs via 5'-epimerization of GlcUA.
• Contains both IdUA-GalNAc and GlcUA-GalNAc disaccharides.
• This substance has wide distribution in tissues and it's primary GAG in the skin, contributing to coagulation, wound repair, and resistance to infection.

Proteoglycan Functions

• As a constituent of extracellular matrix/ground substance, it interacts with collagen and elastin.
• As polyanions, glycosaminaglycans bind polycations, such as Na and K, attracting water by osmotic pressure, contributing to turgor.
• Hyaluronic acid serves as a barrier in tissues, permit passage for metabolites and resist bacteria/infective agents.
• Hyaluronic acid acts as a lubricant/shock absorber in joints; intraarticular injections are used for osteoarthritis pain relief.
• In hormone release, they are in storage/secretory granules releasing contents.
• Hyaluronic acid is at high concentration to promote cell migration in embryonic tissues, helping morphogenesis/wound repair.
• They exist in basement membrand of the kidney's glomerulus for filtration.
• In vitro usage:
• Heparin acts as an anticoagulant.
• In vivo functions:
• Heparin's most important action: is to bind with plasma antithrombin III to impede serine protease thrombin.
• The four naturally occurring thrombin inhibitors: Antithrombin III (75% activity), Α 2-macroglobulin, Heparin cofactor II, and α1-antitrypsin.
• Heparin acts as a coenzyme, increasing activity of Lipoprotein lipase that releases enzymes into circulation.
• Acts as cell receptors, facilitating cell adhesion, cell-cell interactions, on membrane's plasma.
• Give chondroitin sulphates and hyaluronic acid to cartilages for compressibility for weight bearing.
• Dermatan sulphate maintains sclera shape.
• Keratan sulphate I is found in the cornea allowing for corneal transparency

Heteropolysaccharides and Monomers

• Hyaluronic acid contains N-acetyl glucosamine & glucuronic acid.
• Heparin contains sulphated glucosamine & iduronic acid.
• Chondroitin sulphate contains N-Acetylgatactosamine sulphate & glucuronic acid.
• Keratan sulphate contains galactose and galactosamine sulphate.
• Dermatan sulphate contains N-acetylgalactosamine & iduronic acid.

Membrane Lipids

• They include Glycerolipids, sphingolipids, & sterols (cholesterol).
• Other complex lipids: Steroids and isoprenoids.

Sphingolipids: Membrane Lipids

• Include sphingomyelins, neutral gylcolipids, and gangliosides.
• Sphingomyelins include phosphocholine or phosphoethanolamine.
• Neutral glycolipids are cerebrosides and globosides.
• Cerebrosides are 1 sugar.
• Globosides are more than 2 sugars.
• Gangliosides contain sialic acid (Neu5Ac).
• Sterols make up cholesterol found in: animals, plants, fungi.
• Also included are: stigmasterol and ergosterol.

Cholesterol

• It's an important content of all cell membranes and acts as a precursor for steroid hormones (cortisol, testosterone, estrogen, cortisone).
• It is also obtained through biliary cholesterol, intestinal absorption of dietary, or synthesizing acetyl CoA.
• Human bodies can not break down steroid.
• Excess cholesterol becomes other compounds, and is excreted with feces or in other products, such as, such as bile acids/steroid hormones.

Steroids

• Complex molecule of 4 rings.
• Sterols are the most abundant steroids and are steroid alcohols .
• Cholesterol is the best-known steroid due to association in atherosclerosis.
• It a major component of the plasma membrane and plasma lipoproteins, found widely in cells, particularly in nervous tissue.
• Often found as cholesteryl ester, where the hydroxyl group on position 3 is esterified with a long-chain fatty acid, only found in animals.

Steroid Structure

• In animal: Cholesterol is found in bile (chol-bile) and is the major sterol found.
• Cholesterol is 3-hydroxy-5, 6-cholestene.
• A normal 65 Kg adult should have about 200gm.
• The brain and egg yolk are rich, and it is in the spinal chord and neurons.
• Steroids are considered non-saponifiable lipids.

Steroid Function

• Steroids are components of cell membrane and lipoproteins.
• Derivatives of steroid give physiological benefits.
• Vitiman D is derived of 7-dehydrocholesterol, which is derived of cholesterol.
• They are required for the formation of salts, and Bile acids.

Cholesterol Metabolism

• Cholesterol is added to the cholesterol pool from two main sources: dietary intake and synthesis in various body tissues.
• The ability to make hepatic cholesterol is determined by the amount of daily food.
• Its absorption determined by the small intestine is mainly regulated by the absorption efficacy.
• Cholesterol is secreted through biliary cholosterol, or is excreted through feces as bile acid.
• Efflux transporter for the secretion of biliary cholesterol is regulated by sterole ABCG8 and ABCG5.
• Bile acid export pumps regulate the synthesis of of acid via cholesterol ABCB11
• They are lost through digested tract.

Cholesterol Biosynthesis

• Cholesterol is made in 4 stages:
• Condensation of Mevalonate from 3 Acetates
• Conversion of Mevalonate into 2 Isoprenes
• Polymerization of 6 Activated Isoprenes into Squalene
• Cyclization of Squalene, and the modification of Lanosterol.

Cholesterol Biosynthesis: Mevalonate from Acetate

• Reversal process of with final step in B-Oxidation of fatty acids that occurs.
• Aldor condensation located at C3. Reduction of HMG-CoA.
• Commited step In biosphere of isoprenes with reqirement of NADPH for carboxylate reduction to alcohol.

Cholesterol Biosynthesis: Conversion of Mevalonate to Activated Isoprenes

• Requires 3 ATP's In 4 enzymatic steps.

Cholesterol Biosynthesis: Conversions

Polymerization uses Farnesyl-PP.

• Squaleze then requires 1 NADPH
• 2 hydride migrations, and the Modification of Lanosterol gives cholesterol.

Overall Cholesterol Biosynthesis

• 18 ATP Hydrolyzed, Net 27 NADPH.

Cholesterols Derivations and Enzymes

• 7α Hydroxylase and desmolase
• 7α- Hydrcholestrol

Bile (Salts) Acids

• 7α Hydroxylation.
• Must carboxylate using with Coa Amino by glycines and taurines with activated carbsolate.

Biosynthesis and Steroid Production

• Steroid made of of steroidgenic and chondresterol.
• Desmolase comes from P-450 oxidases with 2 oxygen to introdu e diols on C22/C"2.

Vitamin Metabolism

• Is when 7-Dehycholesterol absorbs ultraviolet B.
• This forms with pre-vitamin D3, with vitamin D3.

Isoprenoid Biosynthesis

• Uses Statins, HMG-COA, and Demethellal, and converts them through bile salts.

HMG-CoA and Inhibitors

• Compete and inhibit cholesterol synthesis.

Study Problems: Cholesterol

• The drug known as status helps lowering high lipids, by preventing HMG-COA, and side affects the isoprenoid pathways.