Introduction to Carbohydrates

• Carbohydrates were originally known as "Hydrates of Carbon."
• Carbohydrates are organic substances containing carbon, hydrogen, and oxygen.
• The empirical/general formula for simple carbohydrates is Cn(H2O)n, where n is the number of carbon atoms.
• Carbohydrates are defined as polyhydroxy aldehyde or ketone derivatives or as compounds yielding these derivatives upon hydrolysis.
• Simple carbohydrates have many hydroxyl groups (polyhydroxy) and carbonyl/functional groups as either aldehyde or ketone.

Functions of Carbohydrates

• Carbohydrates are the primary source of energy/fuel for the body, with glucose providing 4 Cal/g.
• They play a structural role, such as glycosaminoglycans in humans, cellulose in plants, and chitin in insects.
• Carbohydrates are precursors for many organic compounds, including fats and amino acids.
• Some carbohydrate derivatives act as drugs, like cardiac glycosides and antibiotics.
• They serve as a storage form of energy; glycogen in animals and starch in plants supply immediate energy.
• Non-digestible carbohydrates, such as cellulose, function as dietary fibers.
• Carbohydrates are constituents of nucleic acids RNA and DNA, including ribose and deoxyribose sugars.
• They play a role in lubrication, cellular intercommunication, and immunity.
• Carbohydrates are involved in detoxification processes, such as glucuronic acid conjugation.

Classification of Carbohydrates

• Carbohydrates are divided into 4 major groups, depending on the number of sugar units.
• Monosaccharides consist of one sugar unit, e.g., glucose and fructose.
• Disaccharides contain two sugar units, e.g., maltose and sucrose.
• Oligosaccharides contain 3-10 sugar units, e.g., raffinose.
• Polysaccharides contain more than 10 sugar units, e.g., starch and cellulose.

Monosaccharides

• Monosaccharides, from Greek words “mono-one, saccharides-sugar," are the simplest group of carbohydrates. They are often referred to as simple sugars.
• Monosaccharides cannot be further hydrolyzed.
• All monosaccharides are reducing sugars and possess strong reducing agents.
• They have the general formula Cn(H2O)n. The common monosacharides are trioses, tetroses, pentoses, and hexoses.
• The classification depends on the number of carbon atoms, e.g., trioses, tetroses, pentoses, and hexoses.
• Further classification relies on whether aldehyde (-CHO) or ketone (-C=O) groups are present, categorizing them as aldoses or ketoses.
• Glyceraldehyde is the simplest carbohydrate. An example of ketotriose is dihydroxyacetone. Exmaples of the hexoses are glucoses and fructoses.

Disaccharides

• Disaccharides contain two molecules of the same or different monosaccharide units.
• Hydrolysis of disaccharides yields two monosaccharide units.
• The two monosaccharide units are joined by a glycosidic bond.
• Lactulose is a synthetic disaccharide with galactose and fructose, neither digested nor absorbed in the intestine.
• Examples of disaccharides are maltose, lactose and sucrose.

Maltose

• Maltose is composed of two α-D-glucose units linked by an α(1→4) glycosidic bond.
• Maltose's free aldehyde group present on C1 of the second glucose enables reducing reactions.
• Dilute acid or the enzyme maltase can hydrolyze Maltose to liberate two molecules of α-D-glucose.

Lactose

• Lactose is a disaccharide found in milk and commonly known as milk sugar.
• β-D-galactose and β-D-glucose compose lactose, connected by a β(1→4) glycosidic bond.
• Lactose exhibits reducing properties, because the anomeric carbon of C1 glucose is free.

Sucrose

• Sucrose (cane sugar) comprises α-D-glucose and β-D-fructose.
• The two monosaccharides are held together by a glycosidic bond (α1 & β2) between C1 of α-glucose and C2 of β-fructose.
• Sucrose is a non-reducing sugar, because the reducing groups of glucose and fructose are involved in the glycosidic bond.

Reducing vs Non-reducing Sugars

• Reducing sugars contain a free or potential (reactive) aldehyde or ketone group.
• Reducing sugars exhibit reducing properties efficiently in alkaline media, reducing metallic ions like Cu++, Bi++, and Fe+++.
• These sugars give positive tests with Benedict's and Fehling's reagents and form osazones.
• Reducing sugars show mutarotation (change in optical activity). Examples include lactose and maltose.
• Non-reducing sugars lack a free or potential aldehyde or ketone group.
• Non-reducing sugars do not show reducing properties and test negatively with Benedict's and Fehling's reagents.
• They do not form osazones and do not exhibit mutarotation. Examples include sucrose and trehalose.

Oligosaccharides

• Oligosaccharides contain 3 to 10 monosaccharide units. An example is maltotriose (glucose + glucose + glucose).

Polysaccharides

• Polysaccharides contain more than ten molecules of monosaccharide units.
• Polysaccharides are classified into homopolysaccharides and heteropolysaccharides.
• Homopolysaccharides (homoglycans) are polymers of the same monosaccharide units.
• Heteropolysaccharides (heteroglycans) are polymers of different monosaccharide units or their derivatives, such as amino sugars and uronic acids.
• Examples of homopolysaccharides are starch, dextrin, glycogen, cellulose, inulin, and chitin.

Starch

• Starch is the primary storage form of carbohydrates in plants and is an important dietary source for higher animals, including humans.
• Starch is a homopolysaccharide composed of α-D-glucose units linked by α(1→4) and α(1→6) glycosidic bonds.
• Starch, also known as glucosan or glucan, contains two forms: amylose (15-20%) in the inner part and amylopectin (80-85%) in the outer part.
• α-amylase (pancreatic or salivary) hydrolyzes starches to produce dextrins, maltose, and glucose units. Amylase acts specifically on D (α-1,4) glycosidic bonds.

Glycogen

• Glycogen serves as the storage form of energy in animals (animal starch; it is mainly present in skeletal muscles and liver).
• The primary glycosidic bond in glycogen is α 1-4, with α 1-6 linkages occurring only at branching points.
• Glycogen is similar to starch's amylopectin component but has more branches. There are 11 to 18 glucose residues between branch points.

Cellulose

• Cellulose is exclusively found in plants and is the most abundant organic substance in the plant kingdom.
• A predominant component of plant cell walls occurs in the absence of animal bodies.
• β-D-glucose units compose cellulose, linked by β(1-4) glycosidic bonds.
• Mammals, including humans, can't digest cellulose due to a lack of an enzyme that cleaves β-glycosidic bonds; α-amylase only breaks α bonds.
• Though indigestible, cellulose is important for human nutrition, as it decreases the absorption of glucose and cholesterol, increases fecal bulk, and acts as a stool softener to prevent constipation.
• Reduced incidence of cardiovascular disease, colon cancer, diabetes, and diverticulosis happens due to cellulose's biomedical effects.

Heteropolysaccharides

• Heteropolysaccharides consists of more than 10 repeating units. Mucopolysaccharides (MPS) or Glycosaminoglycans (GAGs) make up significant part of heteropolysaccharides
• Acidic Non-Sulfated MPS incliude hyaluronic acid
• Acidic Sulfated MPS include heparin, heparan sulfate, chondritin sulfate, dermatan sulfate, and keratan sulfate

Hyaluronic Acid

• Hyaluronic acid is an important GAG in the ground substance of synovial fluid in joints and vitreous humor of eyes.
• Also present as a ground substance in connective tissues and forms a gel around the ovum. Hyaluronic acid works as a lubricant and shock absorbent in joints.

Chondroitin

• Chondroitin 4-sulfate is present in mammalian tissues (bone, cartilage, tendons, heart, valves, skin, cornea etc.).
• Chondroitin 4-sulfate comprises D-glucuronic acid and N-acetyl-D-galactosamine 4-sulfate.

Heparin

• Heparin acts as an anticoagulant (prevents blood clotting) in blood, lung, liver, kidney, and spleen.
• This helps releases the enzyme lipoprotein lipase, clearing the turbidity of lipemic plasma.
• Heparin contains alternating units of N-sulfo-D-glucosamine 6-sulfate and glucuronate 2-sulfate

Dermatan Sulfate

• Dermatan sulfate, mostly in the skin and structurally related to chondroitin 4-sulfate differs through an inversion in the configuration around C5 of D-glucuronic acid to form L-iduronic acid.

Keratan Sulfate

• Keratan sulfate, a heterogeneous GAG with a variable sulfate content and contains small amounts of mannose, fructose, and sialic acid etc., is made of D-galactosamine, N-acetylglucosamine - 6-sulfate.

Structure of Glucose

• The structure of glucose can be represented in the straight chain structural formula (Fisher projection) and in the cyclic formula (Ring structure or Haworth projection).
• Monosaccharides mainly present in ring form when in a solution
• Aldehyde (CHO) or ketone (C=O) groups of monosaccharide when in a solution react with hydroxy (OH) groups to form hemiacetal or hemiketal.
• The aldehyde group of glucose at C-1 reacts with alcohol (OH) groups of C-5 or C-4 to form six-membered rings called glucopyranose or five-membered rings called glucofuranose, respectively.
• The six membered glucopyranose is much more stable than the glucofuranose ring for glucose. In the case of the monosaccharide fructose, the more stable form is fructofuranose.
• Haworth projection formulae are depicted by a six-membered ring pyranose (based on pyran) or a five-membered ring furanose (based on furan).
• Blood Glucose is more thermodynamically stable in ß-D Glucopyranose form.

Optical Activity & Stereoisomerism

• All carbohydrates, except dihydroxyacetone (DHA), possess asymmetric carbon atoms in their structure giving them optical activity and stereoisomerism.
• Asymmetric carbon (chiral) atoms, have four different atoms/groups, result in light rotation ability.
• Optically active substances rotate the plane of polarized light to the right or left and are analyzed via polarimeter tubes.
• Dextrorotatory (d/+) substances rotate the plane of polarized light to the right.
• Laevorotatory (l/-) substances rotate the plane of polarized light towards the left.
• Stereoisomerism arises from chiral carbon atoms.
• Stereoisomers share a chemical and molecular formula and differ only in the spatial configuration.
• Stereoisomers types covered are D and L isomers (enantiomers), epimers, and anomers.

D- and L-isomers

• The orientation of the H and OH groups around the carbon atom just adjacent to the terminal primary alcohol carbon, e.g. C-atom 5 in glucose determines the series.
• On the terminal carbon, when the – OH group is on the right, it belongs to the D-series. On the terminal carbon, when the - OH group is on the left, it is a member of the L-series.

Anomers

• Van't Hoff's rule determines number of isomers as follows: number of isomers= 2n where n is the number of chiral atoms.
• Glucose has 4 asymmetric carbons and 16 isomers.
• Humans use D-glucose or D-sugars with cell enzyme machinery specific to metabolize D-series monosaccharides.
• The hydroxyl group of monosaccharides reacts with its aldehyde or keto functional group for hemiacetal and hemiketal formation.
• Thus, the aldehyde group of glucose at C1 reacts with the alcohol group at C5 to form alpha (α) and beta (β) cyclic hemiacetals.
• Carbon 1 turns asymmetric after ring formation, called an anomeric carbon atom.
• Fischer or Haworth projection formulae represent configurations of glucose in D- and L- isomers.
• The designation α means that the OH-group attached to C-1 is below the plane of the ring,
• The designation β means that it is above the plane of the ring.

Epimers

• Two monosaccharides that differ from each other in their configuration around a single asymmetrically carbon (other than anomeric) atom, are referred to as epimers to each other.
• C-4 epimers include glucose and galactose
• C-2 epimers include glucose and mannose.
• If the glucose and mannose isomers are present at equal concentration, the result is called racmic mixdture or DL mixture
• Racemic mixtures do not exhibit any optical activity, as dextro- and levorotatory activities cancel each other.

Mutarotation

• Mutarotation is the spontaneous gradual change in specific optical rotation to a constant fixed rotation, representing the a and b forms of D-glucose interconversion to an equilibirum state, resulting in 63% b -anomer, 36% a anomer and 1% open chain form.
• Sucrose is dextrorotatory in nature (+66.5°). After hydrolysis, mixture results in levorotatory activity. This is inversion.
• Fructose's optical activity averages −92° and glucose is averages 52.7°. Invertase is the enzyme that digests sucrose,

Monosaccharide Derivatives

• Includes sugar acids, sugar alcohols, deoxysugars, amino sugars and amino sugar acids. Can result in ester formation
• Sugar acids production through the oxidation of an aldehyde group (C1) to COOH results in the formation of gluconic acid.
• Glucuronic acid can be created through the Oxidation of terminal alcohol group (CH2OH)
• The carbohydrate alcohol group requires reduction or ketone modification via a reducing agent for this group to be reduced so it can correspond with a given alcohol.
• Fructose can produce D. sorbitol and D. mannitol.
• Excess presence of Sorbitol and dulcitol in tissues may result in strong osmotic effects, swelling cells, and other pathologist effect like cataracts, peripheral neuropathy or nephropathy. Mannitol is known to reduce intracranial hypertension through forced diuresis.
• Deoxysugars have hydroxyl groups, which act as replacements for hydrogen atom/s. Common for their structure in DNA, like 2-deoxyribose
• Aminosugars are sugars containing an amino group (NH2) to replace hydroxyl groups, such as glucosamine (GluN), galactosamine (GlaN) and mannosamine (ManN).
• The addition of acids via aminosugars for production of aminosugar acids. Neuraminic acid the product of adding pyruvic acid to mannosamine which is then formed to sialic acids.
• N-acetyl neuraminic acid (NANA) in glycolipids and glycoproteins is the product of derivatives of the aminosugar.
• Hydroxyl clusters of monosaccharides generate esters when combined with PHosphoric acids. (ie glucose)