Biochemistry : Epimers, Chirality, and the Anomeric Carbon

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Epimers, Chirality, And The Anomeric Carbon

Mirror-image stereoisomers are called __________.

Possible Answers:

epimers

diastereomers

enantiomers

anomers

Correct answer:

enantiomers

Explanation:

Enantiomers are chiral molecules that are non-superimposable mirror images of each other. Diastereomers result when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent stereocenters and are not mirror images of each other. An epimer is one of two stereoisomers that differ in configuration at only one stereocenter. An anomer is a type of epimer; it is one of two stereoisomers of a cyclic sugar that differs only in its configuration at the hemiacetal or acetal carbon (the anomeric carbon).

Example Question #2 : Epimers, Chirality, And The Anomeric Carbon

Why is it that reducing sugars can be metabolized in humans, but non-reducing sugars cannot?

Possible Answers:

Because only reducing sugars can traverse the cell membrane in order to enter cells where they can be metabolized, whereas non-reducing sugars cannot

Because reducing sugars can bind to the proteins needed for metabolism, whereas non-reducing sugars cannot

Because humans lack the enzyme that degrades beta glycosidic linkages

Because reducing sugars can open their cyclic structure into the straight chain form, whereas non-reducing sugars cannot

Correct answer:

Because reducing sugars can open their cyclic structure into the straight chain form, whereas non-reducing sugars cannot

Explanation:

When it comes to metabolizing sugars, only reducing sugars are able to undergo breakdown. This is because reducing sugars are able to be converted from their closed chain form into their open chain form. It is only in the open chain form that sugars such as glucose can be metabolized.

Only reducing sugars can be converted into their open chain form. The reason for this is that the anomeric carbon for these sugars is not occupied. In their ring form, such sugars exist as hemiacetals that can readily and reversibly undergo chain opening. Additionally, some hemiketals can be converted into their open chain form, but they need to be able to tautomerize into their aldose form first.

Non-reducing sugars have their anomeric carbon tied up in a bond, and thus are locked in an acetal or ketal form. Consequently, they cannot convert into their open chain form, meaning that they cannot be metabolized.

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