Monosaccharides PDF

# Monosaccharides ## Introduction Carbohydrates are a class of molecules consisting of hydrated carbons (ie, carbons that have combined with water in a 1:1 ratio) that plays many important roles in biology. Carbohydrates are a source of energy, help provide cellular structure, assist in protein f...

# Monosaccharides ## Introduction Carbohydrates are a class of molecules consisting of hydrated carbons (ie, carbons that have combined with water in a 1:1 ratio) that plays many important roles in biology. Carbohydrates are a source of energy, help provide cellular structure, assist in protein folding, and are involved in cell-cell recognition and immune responses. Errors in carbohydrate processing can lead to a variety of disease states including degenerative disorders (eg, muscular dystrophy) and connective tissue disorders (eg, Ehlers-Danlos syndrome). The simplest carbohydrates are called monosaccharides. These consist of an uninterrupted carbon chain, in which one carbon is a carbonyl and the other carbons are alcohols (Figure 7.1). - C-OH = Alcohol - C=O = Carbonyl | | | | | | :---------------- | :----------------------------- | :-------------------------- | :----------------------- | | Carbon chain | OH | C=O | Carbon chain | | | C-H | (HC-OH)n | (HC-OH)n | | | CH2 | CH2 | CH2 | | | OH | OH | OH | **Figure 7.1 Structural features of monosaccharides.** This lesson examines the structural features of monosaccharides and gives an overview of their biological functions. ## 7.1.01 Biologically Relevant Monosaccharides A simple monosaccharide is a molecule that meets the following criteria: - It contains at least three carbon atoms. - It has the molecular formula _(CH2O)n_. - it can exist as a linear carbon chain. - one carbon in the linear chain is either an aldehyde or a ketone. - the other carbons in the linear chain are primary or secondary alcohols. Many molecules meet these criteria and are classified as monosaccharides; however, relatively few monosaccharides play significant biological roles. As with the amino acids, memorization of certain biologically relevant monosaccharides is important for the exam. This concept provides an overview of several biologically important carbohydrates and the roles they play. The most abundant monosaccharide in nature is glucose, which has the formula _C6H12O6_. Like most monosaccharides, glucose is given a D or L designation to indicate its stereochemistry (see Concept 7.1.03). Most biologically important carbohydrates can be biochemically synthesized using glucose as the starting material. ## 7.1.02 Monosaccharide Classification Monosaccharides are classified according to the number of carbons they contain, the functional groups within them, and whether they are cyclized. ### Classification by Number of Carbons The empirical formula for a monosaccharide, _(CH2O)n_, can give rise to many molecular formulas depending on the value of _n_. Monosaccharides are classified according to their molecular formula. This general classification uses the Greek prefix for the number of carbons in the molecule followed by the suffix -ose. For example, any monosaccharide with the molecular formula _C3H6O3_ (ie, three carbons) is a triose. The classifications for monosaccharides of various lengths are summarized in Table 7.1. | Number of Carbons _(n)_ | Classification | | :---------------------- | :------------- | | 3 | Triose | | 4 | Tetrose | | 5 | Pentose | | 6 | Hexose | | 7 | Heptose | | 8 | Octose | | 9 | Nonose | | 10 | Decose | **Table 7.1 Monosaccharide classification by number of carbons.** Based on this information, glucose, fructose, mannose, and galactose are hexoses; ribose, ribulose, and xylulose are pentoses: and glyceraldehyde and dihydroxyacetone are trioses. Many enzymes and metabolic pathways are named for the type of carbohydrate on which they act. For example, hexokinase can act on multiple hexoses (eg, glucose, fructose, mannose). Similarly, the pentose phosphate pathway produces and manipulates pentoses with phosphates attached. ### Classification by Anomeric Carbon Position Another important monosaccharide classification relies on the position of the anomeric carbon (ie, the carbon with two bonds to oxygen). In linear form, the anomeric carbon is either an aldehyde or a ketone (Figure 7.5). Monosaccharides that contain an aldehyde are called aldoses, and those that contain a ketone are called ketoses. In aldoses, the anomeric carbon is designated carbon 1. In biologically relevant ketoses, the anomeric carbon is found at carbon 2. | | | | :-------------------------- | :----------------------------- | | **Anomeric carbons** | | | | **OH** | | **Aldose** | **1C-H** | | | _(HC-OH) n_ | | | CH2 | | | **OH** | | | | | **Ketose** | **1CH2** | | | **2C=O** | | | _(HC-OH)n_ | | | CH 2 | | | **OH** | **Figure 7.5 General structures of aldose and ketose sugars.** In their linear form, glucose, galactose, mannose, ribose, and glyceraldehyde all contain aldehydes, making them aldoses. In contrast, fructose, ribulose, xylulose, and dihydroxyacetone each contain a ketone at carbon 2 and therefore are ketoses. Note that ketoses often contain the suffix -ulose. For example, ribulose is the ketose form of ribose, and xylulose is the ketose form of xylose. This naming convention can be helpful in identifying ketoses (although exceptions such as fructose exist). Carbohydrates are often classified by both the number of carbons they contain and the position of the anomeric carbon. For instance, a monosaccharide with six carbons and an aldehyde (eg, glucose) is an aldohexose. A monosaccharide with five carbons and a ketone (eg, ribulose) is a ketopentose. Table 7.2 shows the combined classifications of several biologically important monosaccharides. | | | | | | :---------------------------- | :----- | :------- | :----- | | **Trioses** | **(C3H6O3)** | **(C5H10O5)** | **(C6H12O6)** | | **Aldoses** | | | | | H | C | C | C | | | O | O | O | | H-C-OH | | H-C-OH | H-C-OH | | H-C-OH | | H-C-OH | H-C-OH | | H-C-OH | | H-C-OH | HO-C-H | | H-C-OH | | H-C-OH | H-C-OH | | H | H | H-C-OH | H-C-OH | | D-Glyceraldehyde (aldotriose) | | D-Ribose (aldopentose) | D-Glucose (aldohexose) | | **Ketoses** | | | | | H | C | C | C | | | O | O | O | | H-C-OH | | H-C-OH | H-C-OH | | C=O | | C=O | C=0 | | H-C-OH | | H-C-OH | HO-C-H | | H-C-OH | | H-C-OH | H-C-OH | | H | H | H-C-OH | H-C-OH | | Dihydroxyacetone (ketotriose) | | D-Ribulose (ketopentose) | D-Fructose (ketohexose) | **Table 7.2 Select biologically relevant monosaccharides characterized by number of carbons and position of the anomeric carbon.** ## 7.1.03 Monosaccharide Stereochemistry Some monosaccharides have different molecular formulas (eg., glucose versus ribose). Others have the same molecular formula but different functional groups (eg, glucose and fructose), and are constitutional isomers. However, many carbohydrates have the same molecular formula and the same functional groups at each position. These molecules differ from each other only in their stereochemistry (ie, they are stereoisomers). This concept covers various stereochemical aspects by which monosaccharides can differ. ### L-Sugars and D-Sugars Monosaccharides are classified based on the similarity of their stereochemical configurations to those of L- and D-glyceraldehyde, which are themselves monosaccharides. These designations are assigned based on the configuration of the chiral carbon that is farthest from the anomeric carbon (ie, the second-to-last carbon in the linear chain), as shown in Figure 7.9. Note that the final carbon in any monosaccharide chain is a primary alcohol bound to two H atoms, so it is achiral and does not have stereochemistry. | | | | | | :----------------------- | :------ | :---- | :--- | | **Triose** | **Tetrose** | **Pentose** | **Hexose** | | _(Carbon 2 determines_ | _(Carbon 3_ | _(Carbon 4_ | _(Carbon 5_ | | _L or D)_ | _determines_ | _ determines_ | _determines_ | | | _L or D)_ | _L or D)_ | _L or D)_ | **Figure 7.9 The assignment of L or D stereochemistry is determined by the configuration of the chiral carbon with the highest number.** For monosaccharides, when the last chiral center has an R absolute configuration, it is a D-sugar, and when it has an S configuration, it is an L-sugar (see Organic Chemistry Lesson 3.3 for an explanation of R and S stereochemistry). Note that this fact applies only to unmodified monosaccharides and should not be used to determine whether other molecules are in the L or D form since modifications may alter the Cahn-Ingold-Prelog priority of each substituent. In the Fischer projection of a monosaccharide in linear form, the L isomer shows the hydroxyl group of the last chiral center on the left, whereas in the D form the hydroxyl group is on the right. Importantly, the L and D forms of a given monosaccharide are enantiomers. For instance, in L-glucose every chiral center differs in configuration from that of D-glucose. Flipping only the last chiral center of D-glucose does not convert the sugar to L-glucose but instead forms a monosaccharide called L-idose. To convert D-glucose to L-glucose (or vice versa), every chiral center must be flipped. Figure 7.10 shows several biologically important monosaccharides and their enantiomers. | | | | | | :-- | :-- | :-- | :-- | | **D-Glucose** | **L-Glucose** | **D-Mannose** | **L-Mannose** | | **D-Galactose** | **L-Galactose** | **D-Fructose** | **L-Fructose** | **Figure 7.10 The D and L forms of several monosaccharides.** In contrast to the amino acids (which are predominantly found in the L form), naturally occurring monosaccharides are almost exclusively D isomers. ### Epimers D-glucose, D-mannose, and D-galactose are aldohexoses. These each contain six carbons, each have the anomeric carbon at position 1, and the carbons at positions 2 through 6 each have one alcohol group. These molecules differ from each other by their configurations at some but not all stereocenters, making them diastereomers (see Organic Chemistry Lesson 3.3). Specifically, D-mannose and D-galactose each differ from D-glucose at only one stereocenter. Diastereomers that differ at only one position are called epimers. An understanding of the structure of each sugar is facilitated by memorizing the structure of D-glucose and applying the relationship of the other molecules to it (see Figure 7.11). The Fischer projection of D-glucose shows the hydroxyl groups of carbons 2, 3, and 4 pointing to the right, left, and right, respectively. D-Glucose and D-mannose differ only in the orientation of the hydroxyl and hydrogen groups at carbon 2 (ie, the hydroxyl group of D-mannose points to the left). Because this difference occurs at carbon 2, D-glucose and D-mannose are called C2 epimers. D-Galactose and D-glucose also differ from each other at a signal chiral center, but the difference occurs at carbon 4 instead of carbon 2. Therefore, D-galactose and D-glucose are called C4 epimers (Figure. 7.11). Note that although D-galactose and D-mannose are both epimers of D-glucose, they differ from each other at both carbon 2 and carbon 4. Therefore, D-galactose and D-mannose are not epimers of each other but instead can only be classified as diastereomers. | | | | | :---- | :---- | :---- | | **D-Mannose** | **D-Glucose** | **D-Galactose** | **Figure 7.11 Stereochemical relationships between D-glucose, D-mannose, and D-galactose.** D-ribose has epimers that play biologically important roles, but these epimers are unlikely to be tested on the exam. The Fischer projection of D-ribose can be memorized by recognizing the similarity the word _ribose_ to _"right-bose"_. In D-ribose, the hydroxyl group on every chiral center points to the right in a Fischer projection (see Figure 7.12).