Biochemistry Study Set 13

Biology

Quiz 3 :
Carbohydrates

Quiz 3 :
Carbohydrates

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Raffinose is a sugar that, upon hydrolysis of its glycosidic bonds, yields galactose, glucose, and fructose. The galactose-glucose bond is an ?1?6 linkage, and the trisaccharide is a nonreducing sugar. Draw the structure of raffinose.
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The raffinose is a trisaccharide which yields glucose, galactose and fructose on hydrolysis. It is a non-reducing sugar with no free aldehyde group at 1 st carbon of either glucose or galactose.
The structure of raffinose is as follows:
img

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In the DNA structure, the sugar is completely modified. List each modification and identify an analogous, simpler substitution that exists in another sugar molecule. What sugar properties remain in the DNA molecule?
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In DNA (deoxyribonucleic acid) molecule, the sugar molecule is deoxyribose. Its structure is as follows:
img Modification of DNA sugar: In this sugar molecule, the ribose sugar is deficient of hydroxyl group at position 2 and forms 2-deoxyribose sugar.
Analogous to DNA molecule sugar: There is an analogous sugar derivative glucosamine which is similar in structure to 2-deoxyribose sugar. The difference is only that 2-deoxyribose sugar has five membered ring while glucosamine has six membered ring. They are similar in that both are deficient of hydroxyl group at position 2. But glucosamine has amine group instead of hydroxyl group at position 2 and 2-deoxyribose sugar has no group at position 2. Its structure is as follows:
img The sugar of DNA molecule retains some of the properties of sugars in it. These are water solubility or hydrophobic nature. The 2-deoxyribose sugar has 3 hydroxyl groups which allow it to get easily dissolve in polar solvents like water.

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Glucose in solution has two ring forms and one open-chain form. How many structures exist in equilibrium for a polysaccharide of glucose that has multiple ?1?4 bonds and three ?1?6 bonds?
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Glucose molecule has two ring forms which are alpha and beta. These two ring forms are in equilibrium with each other and an open ring form as follows:
img The structure of alpha and beta form depends only upon the carbon 1. If it is upward, then it is beta form and if downward then alpha form. This is possible only when the carbon at position 1 is free.
If we talk about a polysaccharide in which there are multiple
img bond and three
img bonds. This occurs as follows:
img In the multiple
img bonds, there is only one OH (hydroxyl group) at first position of last glucose of the chain which would be free. In case of
img bond, there would be one hydroxyl group free for each bond. Thus, total 4 hydroxyl groups would be free in the mentioned structure and hence, there would be 4 structures that would exist in equilibrium.

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Reducing sugars are a mixture of alpha and beta forms. Why does one predominate over another in different sugars?
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Related to the previous question, suppose you have equal concentrations of glucose and fructose at room temperature. In both cases, beta-ring forms predominate, but with different percentages. In each case, however, when a reducing sugar measurement is made, the same amount of indicator ion is reduced in each case. Moreover, the amount of directly reacting species for the indicator ion is far less than 1% of the total forms. Explain these findings.
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One indication of the distinction in physical properties between simple sugars (like monosaccharides and disaccharides) and polysaccharides is their observed behavior in solution. For example, only starch can form a paste in water. Speculate on how that can be explained from the structural differences between these types of molecules.
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