Problems - Biochemistry - NCBI Bookshelf
Mutarotation is the change in the optical rotation because of the change in the equilibrium between two anomers, when the corresponding stereocenters interconvert. Cyclic sugars show mutarotation as α and β anomeric forms interconvert. For example, if a solution of β-D-glucopyranose is dissolved in water, its specific. The examples of arabinose and lyxose shown in equation 3 above illustrate this result. Application of the osazone reaction to D-glucose and D-mannose demonstrates The importance of these relationships may be seen in the array of aldose . Consequently, fresh solutions of either alpha or beta-glucose crystals in. Problems. 1. Word origin. Account for the origin of the term carbohydrate. The specific rotations of the α and β anomers of d-glucose are + degrees and.
The reaction follows the same mechanism as regular acetal formation. The bond to the alcohol is given the special name of glycosidic bond. If it is made from the alpha anomer, it is called an apha-glycosidic bond.
If it is made from the beta anomer it is called a beta glycosidic bond. Turning a monosaccharide into an acetal has an important consequence. The anomers do not interconvert without strong acid so the alpha anomer will stay alpha and the beta anomer will stay beta. Shown below is methyl beta-D-glucopyranose, the methyl acetal of glucose with a beta glycosidic bond.
Specific enzymes can put carbohydrates together by making glycosidic bonds between the anomeric carbon of one carbohydrate and one of the -OH groups on another carbohydrate. A furanoside reacts with only one molecule of periodate; formate is not formed. Identify the following four sugars. A trisaccharide unit of a cell-surface glycoprotein is postulated to play a critical role in mediating cell-cell adhesion in a particular tissue. Design a simple experiment to test this hypothesis.
The trisaccharide itself should be a competitive inhibitor of cell adhesion if the trisaccharide unit of the glycoprotein is critical for the interaction.
Each of the hydroxyl groups of glucose can be methylated with reagents such as dimethylsulfate under basic conditions. Explain how exhaustive methylation followed by compete digestion of a known amount of glycogen would enable you to determine the number of branch points and reducing ends.
Reducing ends would form 1,2,3,6-tetramethylglucose. The branch points would yield 2,3-dimethylglucose. The remainder of the molecule would yield 2,3,6-trimethylglucose.
Raffinose is a trisaccharide and a minor constituent in sugar beets. No open-chain forms are possible. A taste of honey. The furanose form is the more stable form. Draw the two forms and explain why it may not always be wise to cook with honey. Heating converts the very sweet pyranose form to the more stable but less sweet furanose form.
Fructose can react with hydroxyl group to form a hemiketal group, and it can formed pyranose or furanose depending on whether the C-2 keto group reacts with the C-6 or C-5 hydroxyl group. D-Fructose is the most common ketohexose. Ketoses in Reactions[ edit ] Transketolase Reaction[ edit ] The Transketolase reaction is very similar to the Transaldolase reaction.
However, the Transketolase is different because it transfers a two carbon unit instead of Transaldolase's three carbon unit. Thiamine pyrophospate TPP ionizes so that it has a carbanion which is a negatively charged carbon. The importance of carbanion is that they can attack carbonyls, so that carbons are added in a sense to the nucleophile. TPP attacks a ketose substrate where it than releases the aldose product to yield an activated glycoaldehyde unit. An activated glycoaldehyde unit is an electron sink because of a positively charged nitrogen atom where a carbonyl of an aldose product is attacked and then separated after some electron movement.
The importance of the transketolase reaction is that it is the mechanism that the enzyme TPP uses to change a ketose substrate to a ketose product that has a different group attached to it.
Transaldolase Reaction[ edit ] The transaldolase reaction involves the transfer or a three carbon dihydroxyacetone unit from a ketose donor to an aldose acceptor.
Unlike the transketolase reaction, transaldolase does not contain a prosthetic group; instead the reactions begins with a Schiff base formed between the carbonyl group of the ketose substrate and the amino group of a lysine residue at the active site of the enzyme. Next the Schiff base is protonated and the bond between C-3 and C-4 break which releases the aldose product. The leftover negative charge on the Schiff-base carbanion is stabilized by resonance while the positive charge on the nitrogen atom of the protonated Schiff base acts as the electron sink.
The Schiff-base remains stable until a suitable aldose becomes bound which allows the dihydroxyacetone to react with the carbonyl group of the aldose and the ketose product is released from the lysine side chain via hydrolysis of the Schiff-base.
Transaldolase is a target of autoimmunity in patients with multiple sclerosis which is the selective destruction of oligodendrocytes that selectively expresses transaldolase in the brain. Ketose in the Calvin Cycle[ edit ] The Calvin cycle, or dark reactions, is one of the light-independent reactions. In the third phase of the this reaction, a five-carbon sugar is constructed from six-carbon and three-carbon sugars.
A transketolase and an aldolase are the major factors in the rearrangement. The transketolase, which is in the pentose phosphate pathway, requires a coenzyme, thiamine pyrophosphate TPPto transfer a two-carbon unit from a ketose to an aldose.
Whereas the transaldolase transfers a three-carbon unit from a ketose to an aldose. In summary, transketolase first converts a six-carbon sugar and a three-carbon sugar into a four-carbon sugar and a five-carbon sugar.
Then, aldolase combines the four-carbon product and a three-carbon sugar to form the seven-carbon sugar. This seven-carbon sugar then finally reacts with another three-carbon sugar to form two additional five-carbon sugars. Energy for Organic Organisms[ edit ] Glucose C6H12O6 is one of the main products of the photosynthetic process by plants that initiates the cellular respiration process that produces ATP adenosine triphosphatethe basic energy currency for prokaryotes and eukaryotes.
Glucose is also involved in the energy-harvesting process of glycolysis, which converts glucose into pyruvate and yields a much lesser amount of ATP than is produced by the electron transport chain within cellular respiration. Glucose is an essential source of energy for the body. Modified monosaccharides[ edit ] One example of modified monosaccharides are the phosphorylated sugars.
An important phosphorylated sugar is glucose 6-phosphate, which is a glucose phosphorylated on carbon 6. The significance of this molecule is that it provides energy in certain metabolic pathways, and it can be converted and stored as glycogen when blood glucose levels are high. If blood glucose levels are low, glucose 6-phosphate can be converted back into glucose to enter the bloodstream once again.
A unique property of glucose 6-phosphate is that once glucose is phosphorylated, the sugar possesses a negative charge. This prevents the molecule from leaving the lipid-bilayer membranes. This allows the cell to easily access the modified sugar to provide energy for metabolic pathways such as glycolysis, or convert it to glycogen as storage.
Importance of Carbohydrates in Nature[ edit ] The biological significance of carbohydrates is unquestionable in the natural world with its essential roles in providing metabolic energy. Carbohydrates not only serve roles in energy, but also storage and plant cell wall structure; however carbohydrates are also involved in a variety of biological processes including the immune response, cell—cell interactions, fertilization, viral infection, and drug efficacy, among others.
In recent years, researchers are discovering and understanding new sugar moieties that may have important ramifications for the development of new therapeutics. For example, the dideoxysugar and trideoxysugar moieties that are synthesized by a wide range of bacteria, fungi, and plants are representation of a captivating class of carbohydrates.