5. Bichemistry Carbohydrates PDF

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Biochemistry Carbohydrates Study Guide CARBOHYDRATES – MONOSACCHARIDES AND DISACCHARIDES ● Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. o Many, but not all, carbohydrates have the empirical formula (CH2O)n ▪ Some also contain nitrogen, phosphorus, or sulfur. o Three major size classifications for carbohydrates ▪ Monosaccharides ▪ Oligosaccharides ▪ Polysaccharides MONOSACCHARIDES ● Monosaccharides o Can either be aldehydes or ketones with two or more hydroxyl groups. ▪ Many of the carbon atoms to which hydroxyl groups are attached are chiral centers, which give rise to many sugar stereoisomers. ● Number of Stereoisomers equals 2n where “n” is the number of chiral centers.

Glyceraldehyde and dihydroxyacetone are the simplest of the carbohydrates, only having a total of three carbons, making it the smallest unit for saccharides. ● Monosaccharides have two families: Aldoses and Ketoses. o Aldoses are monosaccharides that contain an aldehyde functional group, where the carbonyl will be at the end of the carbon chain (See Glyceraldehyde) o Ketoses are monosaccharides that contain a ketone functional group, where the carbonyl will be in any position except the end of the carbon chain (See Dihydroxyacetone) ● To characterize a monosaccharide, one should count the number of carbons within the backbone and determine if the carbonyl group is either an aldehyde or a ketone.

o Monosaccharides with three, four, five, six, and seven carbons within their backbone will receive the following suffixes respectfully. ▪ -triose, -tetrose, -pentose, -hexose, and -heptose ▪ The prefix of the name will either be aldo-, or keto- depending on the carbonyl functional group. ● As stated before monosaccharides are chiral o Mainly exist in D form because that is most prevalent in biological systems.

Two different forms of Glyceraldehyde, to determine D or L for a monosaccharide look at the penultimate carbon, and if the hydroxyl group is to the right, then the monosaccharide is in the D conformation. If the hydroxyl group is to the left, then the monosaccharide is in the L conformation. D and L monosaccharide are enantiomers being that they are non-superimposable mirror images. (D and L do not speak for the overall optical activity of the carbohydrate such as (+) and (-)) Using this information about chirality and the 2n rule, we can more easily understand and characterize the D and L aldohexoses (such as glucose and company).

Because there are four chiral centers for all of the major aldohexoses, using the 2n rule we know that there is a total of 16 different stereoisomers for aldohexoses. We also know that half of those must be the D form and the other half must be the L form. Therefore, we can infer that the D-aldohexoses will be diastereomers of each other, and the L-aldohexoses will also follow suit. All the above D-aldohexoses do not differ at every chiral center. They differ only at 1, 2, or 3, chiral centers. There is a special term to be used to describe carbohydrates that differ in configuration at ONE chiral center, that is epimers. The boxed carbohydrates above are the

epimers of D-Glucose, with D-Mannose being a C-2 epimer and D-Galactose being a C-4 epimer. Epimers are technically diastereomers that only differ at one chiral center.

● Similarly, to aldohexoses there are ketohexoses o The ketohexoses only have 3 chiral centers o QUESTION: How many stereoisomers are there for ketohexoses? How many of those are D and how many of those are L? ● One of the common ketohexoses, the constitutional isomer of glucose is fructose o Fructose is a fruit sugar that when bound to glucose creates sucrose (a disaccharide) o Fructose is absorbed directly into the blood stream during digestion o Fructose Corn Syrup ▪ Alternative to sucrose (more on sucrose later) ▪ Same formula, differs slightly in the structure ▪ Sucrose (50:50), while HFCS (55:43) 3% larger sugar molecules.

The leftward image depicts the four different D-ketohexoses. Similarly, the other four ketohexoses are the enantiomers of the ones shown. Can you name the epimers of D-Fructose? Which ones are they? What about the diastereomers?

Before moving on two cyclization of carbohydrates it is important to understand Fischer projections and how to convert from either the Fischer projection to the line-stick model or vice versa. You shouldn’t have to use a model kit or anything to convert these structures.

Look at the carbonyl carbon first, and if it is in the up position like it is here then follow these

rules to determine if the hydroxyl groups of the chiral centers will be on the left or the right of the Fischer projection. For carbons in the down direction (C-2 and C-4) hydroxyls with wedge bonds should be on the left of the Fischer projection, while dashed bonds should be on the right of the Fischer projection. For carbons in the up direction (C-3 and C-5) hydroxyls with a wedge bond should be on the right, while dashed bonds should be on the left. If the carbonyl carbon is in the down position, then just switch the roles of the down and up rules/tricks! This gets super easy if you practice a bunch. ● Cyclic structures of monosaccharides o The formation of the cyclic ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals. ▪ Aldehyde and ketones carbon is electrophilic, while alcohol oxygen is nucleophilic.

An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond. The reaction can occur in one of two ways because the alcohol molecule can either attack the “front” or “back” of the carbonyl carbon. The reaction can therefore produce either of two stereoisomeric configurations, denoted as α or β. Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers. And the carbonyl carbon atom is called the anomeric carbon. ● When a sugar cyclizes a six-membered, or five-membered ring can form. o Pyranoses are the six-membered rings o Furanoses are the five-membered rings ● Haworth projections are used to accurately display the structure of cyclic sugars

Cyclization of ribose to a variety of isomers. The left path showing the formation of a sixmembered ring (pyranose), while the left shows the formation of a five-membered ring (furanose). Additionally, remember that the alpha and beta confirmations can exist at the same time. The five membered ribose is more favorable due to selectivity for phosphorylation reaction. ● Mutarotation is the process of a cyclic sugar to open and close causing a switch in the α and β confirmations while in an aqueous solution. o A solution of β-D-Glucose and a solution of α-D-Glucose eventually form identical equilibrium mixtures having identical optical properties. ▪ This mixture contains about one-third α-D-Glucose, two-thirds β-DGlucose, and small amounts of linear glucose and glucofuranose.

Haworth projections only tell us about the stereochemistry of a cyclic sugar as a planar configuration and assumes either of two “chair” conformations. The configuration of alpha and

beta forms of sugars requires the breakage of bonds, while changes in the configuration of conformers does not. The specific three-dimensional structures of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides. Remember when you flip a chair confirmation just switch equatorial substituents to axial and vice versa.

Be able to convert a Fischer projection to the corresponding Haworth convention, practice makes perfect so make sure to practice doing this. Here is how to do it. To convert the Fischer projection formula of any linear D-hexose to a Haworth perspective formula showing the molecule’s cyclic structure, draw the six-membered ring (five carbons, and one oxygen at the upper right), number the carbons in a clockwise direction beginning with the anomeric carbon, then place the hydroxyl groups. If a hydroxyl group is to the right in the Fischer projection, it is placed pointing down (below the plane) in the Haworth projection; if it is to the left in the Fischer projection, it is placed pointing up (above the plane) in the Haworth projection. The terminal —CH2OH group projects upward for the D enantiomer, and downward for the L enantiomer. The hydroxyl on the anomeric carbon can point up or down. When the anomeric hydroxyl of a D-hexose is on the same side of the ring as C-6, the structure is by definition β; when it is on the opposite side of C-6, the structure is α. DERVIATIVES OF SUGARS ● Organisms contain a variety of hexose derivatives o Oxidation of the C-1 carbon produces aldonic acids ▪ EX: Glucose would become Gluconate or gluconic acid o Oxidation of the C-6 carbon produces uronic acid ▪ EX: Glucose to Glucuronic acid, Galactose to galacturonic acid, etc. o Sialic acids are a family of sugars with the same nine-carbon backbone. ▪ EX: N-acetylneuramic acid is a derivative of N-acetylmannosamine which is found in many glycoproteins and glycolipids. ● Sugar phosphates are important derivatives. o Important derivatives for metabolism.

o Sugar phosphates are relatively stable at neutral pH and bear a negative charge. One effect of sugar phosphorylation within cells is to trap the sugar inside the cell. o Several important phosphorylated derivatives of sugars are components of nucleotides. ▪ EX: Uridine diphosphate glucose – nucleotide sugar activated form of glucose glycogen precursor. ● Deoxy sugars are also important derivatives o Replacement of a hydroxyl group with a hydrogen. ▪ EX: Deoxyribose (2-Deoxy-D-Ribose) is a DNA building block ▪ EX: L-Fucose (6-Deoxy-L-Galactose) is part of the ABO blood group antigen. DISACCHARIDES ● Disaccharides consist of two monosaccharides joined covalently by an O-Glycosidic bond. o O-Glycosidic bond formed when a hydroxyl group of one sugar molecule, typically in its cyclic form, reacts with the anomeric carbon of the other. ▪ This reaction represented the formation of an acetal from a hemiacetal and an alcohol, the resulting component is called a glycoside. ▪ Glycosidic bonds can be readily hydrolyzed with acid, but not base. o N-Glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

A disaccharide is formed from two monosaccharides when an –OH of one monosaccharide molecule condenses with the intramolecular hemiacetal of the other, with elimination of water and formation of a glycosidic bond. The reversal of this reaction is hydrolysis—attacked by H2O on the glycosidic bond. The maltose molecule, shown here, retains a reducing hemiacetal at the C-1 not involved in the glycosidic bond.

Pictures of more common disaccharides, as we see above maltose is composed of an alpha 14 linkage between two glucose molecules. Lactose is composed of a beta 1-4 linkage between galactose and glucose. Sucrose is composed of a alpha 1-2 beta linkage between glucose and fructose. Trehalose is formed between an alpha 1-1 alpha linkage between two glucose molecules. Take note on how glycosidic bonds are different from each disaccharide and be able to name the short hand versions. Also, know that Maltose and Lactose are both reducing sugars, while Sucrose and Trehalose are not.

Sometimes the anomers can exist for some of these bonds remember that the alpha and beta conformations for each monosaccharide exist within aqueous solutions. Be aware of what sugars are there and make sure you have structure of glucose memorized. REDUCING SUGARS ● Reducing sugars are sugars that can reduce cupric ions. o These sugars are oxidized at the anomeric carbon position o All monosaccharides are reducing sugars because they have the available hydroxyl group at the anomeric carbon. o Disaccharides and so on must be looked at more closely. ▪ If the disaccharide contains a free anomeric hydroxyl then it is a reducing sugar ▪ If both the anomeric carbons are participating in the glycosidic bond then the sugar is nonreducing ● Benedicts test o Originally detected by their ability to reduce Cu2+ and Ag+ o Identify differences between reducing and nonreducing sugars.

Test showing the results of a Benedicts test, where a blue solution indicates no reducing sugars, green indicates small amounts of reducing sugar, orange indicates a moderate amount, and red indicates high levels of reducing sugars.

Reducing sugars are the basis of the Fehling’s reaction, a semiquantitative test for the presence of reducing sugar that for many years was used to measure elevated glucose levels in people with diabetes mellitus. The blood sample containing glucose is oxidized, which through the help of glucose oxidase allows for the reduction of O2 to H2O2. Additionally, the redox reaction of the mediators allows for the emittance of an e- which due to lower redox potential allows the pickup of the electron from the redox surface.

OLIGOSACCHARIDES ● Oligosaccharides range in complexity from 3-20 branched or unbranched sugar residues. o Human milk contains oligosaccharides that act as probiotics and protect against microbial infections.

● Related oligosaccharides are modifications of the same disaccharides o Probiotics ▪ Probiotics allow “good” bacteria to out compete bad bacteria. ● Bifidobacterium is an example. POLYSACCHARIDES ● Polysaccharides are also known as glycans. o Differences between polysaccharides are from the monosaccharides unites, the length of their chains, the types of bonds, and the degree of branching. o Two types ▪ Homopolysaccharides – Contain only a single monomeric species ▪ Heteropolysaccharides – Contain two or more different kinds

Polysaccharides may be composed of one, two, or several different monosaccharides, in straight or branched chains of varying length. ● Some Homopolysaccharides are storage forms of fuel, such as starch and glycogen. ● Starch stores glucose in plants and fungi and contains two types of glucose polymers ▪ Amylose ▪ Amylopectin o Amylose consist of long, unbranched chains of D-glucose residues connected by (α1→4) linkages (as in maltose). ▪ Chains vary in molecular weight from a few thousand to a few million. ▪ Contains 100-1000 D-glucose units

Structure of amylose, which is a component of starch within plants and fungi.

o Amylopectin has a high molecular weight (up to 200 million) but unlike amylose is highly branched. ▪ Glycosidic linkages joining successive glucose residues in amylopectin are (α1→4); the branch points, occur every 24 to 30 residues, are (1α→6) linkages. ▪ Contains anywhere from 300-6000 glucose units

Left picture showing structure of amylopectin, right picture showing a cluster of amylose and amylopectin like that believed to occur in starch granules. Starch amylopectin (black) form double-helical structures with each other or with amylose strands (blue). Amylopectin has frequent (α1→6) branch points (red). Glucose residues at the nonreducing ends of the outer branches are removed enzymatically during the mobilization of starch for energy production. ● Starch can be digested by humans using an enzyme called amylase ● Glycogen is the main storage polysaccharide of animal cells o Glycogen is similar to amylopectin in the sense that it is a polymer of (α1→4)linked glucose subunits, with (α1→6)-linked branches. o However, Glycogen is more extensively branched (every 8 to 12 residues), and is more compact than starch o Glycogen contains about 50,000 glucose units o 7-10% w/w liver 2% muscle

Because each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule with n branches has n + 1 nonreducing ends, but only one reducing end. When glycogen is used as an energy source, glucose units are removed one at a time from the nonreducing ends. Degradative enzymes that act only at nonreducing ends can work simultaneously on the many branches. ● Dextran are bacterial and yeast polysaccharides made up of (α1→6)-linked poly-DGlucose. o All have (α1→3) branches, and some also have (α1→2) or (α1→4) branches. o Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans, which are adhesive and allow the bacteria to stick to teeth and to each other. ▪ The dental plaque has components of sucrose and dextrans ▪ The dextrans provide energy to the bacteria that adhere to them. ● Some Homopolysaccharides serve structural roles ● Cellulose is a tough, fibrous, water-insoluble substance found in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body. o Like amylose, cellulose is a linear, unbranched homopolysaccharide. ▪ Difference from amylose lies in the configuration of the glucose molecules that compose cellulose. ▪ The D-Glucose is in the β configuration for cellulose (β1→4), different from the α configuration of amylose (α1→4). o Cellulose cannot be digested within the diet due to the (β1→4) linkages. ▪ Termites can readily digest cellulose due to a symbiotic relationship with Trichonympha which secretes cellulase and allows for the breakdown of cellulose. ▪ Cows also can utilize the energy provided by cellulose through symbiotic relationships with microorganisms within the rumen. ▪ Breakdown of cellulose forms acetate, propionate, and β-hydroxybutyrate for milk projection.

The difference in the linkage between cellulose and amylose, allows cellulose pictured above to fold differently in space, giving a very different macroscopic structure and physical properties. Hydrogen bonding that occurs between adjacent chains allows for higher levels of packing which makes it useful in commercial products such as cardboard and insulation material. ● Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in (β1→4) linkage. o Chitin differs from cellulose that the hydroxyl group of C-2 is replaced with an acetylated amino group. o Chitin forms extended fibers like cellulose and cannot be digested by vertebrates. ▪ Chitin is the principle component of exoskeletons and is the second most abundant polysaccharide behind cellulose.

A short segment of chitin, a homopolymer of N-acetyl-D-glucosamine units in (β1→4) linkage. ● Bacterial and Algal Cell walls contain structural Heteropolysaccharides. ● The rigid component of bacterial cell walls, peptidoglycan, is a heteropolymer of alternating (β1→4)-linked N-acetyl-glucosamine (NAG) and N-acetylmuramic acid (NAM) residues. o The linear polymers lie side by side in the cell wall, cross-linked by short peptides, exact structures depend on bacterial species. o Peptides form on the NAM subunits. o Peptidoglycan can be hydrolyzed by lysozymes

Left is the structure of the peptidoglycan structure found in bacterial cells, and the right shows the core NAG and NAM component structures.

● Another heteropolysaccharide are the glycosaminoglycans, which are a family of linear polymers composed of repeating disaccharide units. o Glycosaminoglycans are unique to animals and bacteria and not found in plants o One of the two monosaccharides is always either N-acetylglucosamine, or Nacetylgalactoseamine. o The other monosaccharide is either D-glucuronic or L-iduronic acid ● Here are some of the common glycosaminoglycans of the extracellular matrix (ECM) o Hyaluronan, also known as hyaluronic acid o Chondroitin Sulfate o Keratan sulfates o Heparan Sulfate NOTE: there are more types of glycosaminoglycans however these are the main ones we focused on in class. ● Hyaluronan, hyaluronic acid, contains alternating residue of D-glucuronic acid and Nacetylglucosamine o Hyaluronan forms clear, highly viscous, noncompressible solutions that serve as lubricants in the synovial fluid o...