Carbohydrates and Nucleic Acids Help (page 2)
Carbohydrates (saccharides) are aliphatic polyhydroxyaldehydes (aldoses), polyhydroxyketones (ketoses), or compounds that can be hydrolyzed to them. The suffix -ose denotes this class of compounds. The monosaccharide D-(+)-glucose, an aldohexose, is formed by plants in photosynthesis and is converted to the polysaccharides cellulose and starch. Simple saccharides are called sugars, and typically have molecular formulae of Cn(H2O)n. Polysaccharides are hydrolyzable to monosaccharides. A carbohydrate is classified as either a ketose (if it contains a ketone carbonyl group) or an aldose (if it contains an aldehyde carbonyl group).
Fisher Projections. A simplest carbohydrate is glyceraldehyde. It is an aldotriose, since it is a 3-carbon aldehyde. It is common to draw carbohydrates in a Fisher projection, in which the carbon chain is drawn vertically on the page (aldehyde at top), and each stereogenic center is arranged so that the carbon chain is oriented back into the page. The substituents (hydrogen and hydroxyl) are drawn horizontally. The example below is D-glyceraldehyde.
Glucose is an aldohexose, so it is a 6-carbon aldehyde. The naturally occuring enantiomer, D-glucose, is shown here. The designation D stems from the last stereogenic center in the chain. If in the Fisher projection, the hydroxyl of the last stereogenic center (in the box) is on the right side of the chain, it is designated D. If that hydroxyl is on the left, the molecule is an L-sugar.
Mutarotation. Nearly all carbohydrates are chiral, so they rotate planepolarized light either clockwise (+) or counterclockwise (–). Naturally occurring (+)-glucose is obtained in two forms: mp = 146 °C, [α]D = +112° and mp = 150 °C, [α]D = +19°. The specific rotation of each form of glucose changes (mutarotates) in water, and both reach a constant value of +52.7°. This phenomenon stems from the formation and equilibration of intramolecular hemiacetals. Carbohydrates that form 5-membered ring acetals are known as furanoses and ones that form 6-membered ring acetals are known as pyranoses.
The two stereoisomeric hemiacetals are known as anomers, and they differ in configuration at the new stereogenic center formed from the carbonyl carbon. The anomer with the new hydroxyl "up" is known as the β-anomer, while, the anomer with the new hydroxyl "down" is the α-anomer.
Chemical Reactions of Monosaccharides
The chemistry of monosaccharides is the chemistry of the component aldehydes, ketones, and alcohols that make up these molecules. One example of how the chemistry of carbohydrates is the same as the chemistry of ketones is in their reaction with NaBH4.
Reduction to Alditols. Both ketoses and aldoses can be reduced with NaBH4 to reduced forms, known as alditols. With ketoses, this often produces 2 diastereomeric alditols.
Disaccharides are acetals in which an OH of one monosaccharide (the aglycone, denoted A) is bonded to the anomeric C of a second monosaccharide, B. The disaccharide is a glycoside of B, involving the anomeric center of one carbohydrate and an alcohol (in this case another carbohydrate). For example, the α-1,4-linked glucose dimer below is maltose. Polysaccharides are sugar polymers, linked in the same kind of manner.
The nucleic acids, RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), are carbohydrate biopolymers with phosphate backbones. The repeating sugar in RNA is ribose, and in DNA it is 2-deoxyribose.
Nucleosides are glycosides of ribofuranose or deoxyribofuranose. Ribofuranose is the monosaccharide ribose in the furanose form. The nitrogenous bases (below) are bonded to the anomeric carbon (C1) of the sugar.
Adenine and guanine are known as the purine bases. Cytosine, thymine, and uracil are the pyrimidine bases.
Nucleotides are phosphate esters of nucleosides, formed at the CH2OH group of the sugar. Adenosine triphosphate (ATP) is the triphosphate ester of adenosine, formed from adenine and ribose.
Nucleic acids are polymeric nucleotides in which phosphate esters link ribose or deoxyribose molecules through the C1–OH of one and the C3–OH of the other. In RNA, the aglycone nitrogen bases are cytosine, adenine, guanine, and uracil. In DNA, thymine replaces uracil. The RNA polymer is like that of DNA, except that in DNA an H replaces the OH group on C2 of the ribose ring.
DNA, a constituent of the cell nucleus, consists of two strands of polynucleotides that are coiled to form a double helix. The strands are held together by H bonding between the nitrogen bases. The pyrimidines always form H bonds with a specific purine; i.e., cytosine with guanine and thymine with adenine. However, in RNA the pairing is between uracil and adenine.
Through its sequence of nitrogen bases, DNA stores the genetic information for cell function and division, and for biosynthesis of enzymes and other essential proteins. In protein synthesis, the information in the DNA is transcribed onto messenger RNA (mRNA), which moves from the nucleus to the ribosomes in the cytoplasm of the cell. Here the information is transferred to ribosomal RNA (rRNA). Transfer RNA (tRNA) carries amino acids to the surface of the rRNA, where the protein is "grown." A specific three-term sequence of bases in the mRNA, called a codon, calls up a tRNA carrying the specific amino acid that is to be the next unit in the growing protein chain. For example, the codon cytosine. uracil. guanine is translated as "leucine."
The existence of 64 – 20 = 44 excess codons allows a valuable redundancy in the genetic code. It also permits the signaling for the start and end of the protein chain.
Practice problems for these concepts can be found at: Carbohydrates and Nucleic Acids Practice Problems
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