Carbohydrates (with interactive animations)

Carbohydrates    

Prof. Doutor Pedro Silva

Associate Professor, Universidade Fernando Pessoa

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Metabolic pathways:

Carbohydrates (also known as sugars) are biological molecules composed by carbon, oxygen and hydrogen in 1:1:2 ratios. In simple carbohydrates (monosaccharides) with n carbons there are:
  • one carbonyl group - Terminal carbonyls (i.e., aldehydes) yield aldose sugars, whereas medial carbonyls (i.e. ketones) yields ketose sugars.
  • n-1 hydroxyl groups, each bound to a different carbon atom:


    As shown above, each monosaccharide may contain n-2 (in aldoses), or n-3 (in ketoses) assymetric carbons, i.e. carbon atoms bearing four different substituents. The presence of assymetric carbons usually entails the non-superposability of a molecule with its mirror image, as shown in the example below:


    Therefore, every sugar may can have several isomers. Two sugars with the same composition are enantiomers if they are non-superposable reflexions of each other. Otherwise, they are diasteroisomers. Epimers are a special case of diasteroisomers which differ only in the configuration of one of their assymetric carbons:


    In the example above, 1 and 2 are enantiomers, 1 and 3 são diasteroisomers, 2 and 3 are epimers. Monossaccharide structures are usually written as Fischer formulae, a special representation drawn along the carbon chain with the substituents facing the observer and the chemical symbols of the assymetric carbons and associated hydrogens are ommited. The Fischer formulae of the sugars drawn above are:


    a glucose molecule is depicted below (left). Its Fischer formula is shown at the right side.

    Naturally-occurring monossaccharides have one common feature: when written as Fischer formulae, the las assymetric carbon always has an OH group at its right side . These sugars (e.g. sugar (1) above) are called D-sugars (from Latin dexter, meaning "right"), and their enantiomers are L-sugars (from Latin levo, meaning "left"), e.g. sugars (2) and (3) above.

    A monossaccharide may be converted into cyclic forms, through nucleophilic attack of its carbonyl group by one of its hydrocil groups. This reaction is a relevant example of hemiketal occurrence in Biology. These cyclic forms are only stable if the ring has five or more sides (otherwise the bond angles will be too different from the ideal value of ≈ 109o ). In practice, common sugars form either five-sided (furanoses) or six-sided rings (pyranoses). In aqueous solution, cyclic forms are often much more stable than the corresponding linear forms.


    Two cyclic forms of a pyrnose or furanose are possible, due to variable orientation of the carbonyl group prior to the nucleophilic attack. The hemiketal carbon (usually dubbed anomeric carbon) arising from the carbonyl group may therefore have an OH group in two different orientations: if this OH lies on the same side (in a Fischer proejction) as the OH in the last assymetric carbon, it is a α form ; otherwise, it is a β form. For D-hexopyranoses, this means taht in a α form, the OH group at the anomeric carbon and the CH2OH at the last assymetric carbon lie on opposite sides of the ring.

    Dissaccharides

    Two monosaccharides may be chemically joined through a glycosidic bond. In this reaction, the OH at an anomeric carbon attacks a carbon atom at another sugar molecule, thereby releasing the OH that had been bound to that carbon:


    Since this sugar molecule contains several OH goups, several glycosidic bonds are possible. The glycosidic bond depicted above joins carbon 1 of sugar A to carbon 4 of sugar B. Furthermore, the anomeric carbon involved in the glycosidic bond has α configuration, and the glycosidic bond is therefore a α-1,4 bond.
    (Glucose α-1,4 glucose) the product of starch degradation by amylases.
    (Glucose α-1 β-2 fructose) the sugar extracted from sugar-cane.
    (Galactose β-1,4 glucose) the sugar present in milk. In most mammals, the enzyme responsible for lactose hydrolysis (lactase) is only synthesized during nursing. Absent lactase, lactose cannot be digested, and it becomes an abundant food source for gut bacteria, thereby causing nausea, vomitting, and diarrhea. Since cattle domestication, several human populations have developped the ability to express lactase throughout adult life.

    Some important polissaccharides

  • Cellulose – linear polymer of glucose molecules joined by β-1,4 glycosidic bonds.

    The mechanical resistance of cellulose arises from its conformation: the occurrence of every polar group in equatorial positions allows the formation of many H-bonds between two parallel molecules of cellulose.
  • Amylose – linear polymer of glucose molecules joined by α-1,4 glycosidic bonds. It has an helix structure and is water-soluble. It is a component of starch.
  • Amylopectin – Also a component of starch. It is a branched polymer of glucose molecules joined by α-1,4 glycosidic bonds and α-1,6 bonds (in the branching points). Branching points are relatively distant (around 20-30 glucose residues) from each other.
  • Glycogen – Similar to amylopectin, but present in animal cells. It has many more branhching points (around every 10 glucose residues), and it contains a bound protein (glycogenin) in its reducing end.

    Further reading

    cover Biochemistry, by Donald Voet & Judith Voet

    An excellent text. It presents Biochemistry with frequent references to organic chemistry and biochemical logic. Highly reccommended for students of Biochemistry, Chemistry and Pharmaceutical Sciences.

    cover Biochemistry, Stryer

    A widely used classical text, frequently updated and re-issued.

    cover Textbook of Biochemistry with Clinical Correlations, Thomas Devlin

    Strongly advised to students in Nursing, Medicine, Dentistry, etc. Plenty of examples of application of biochemical knowledge to clinical cases.

    cover Principles of Biochemistry, Lehninger

    A widely used classical text, frequently updated and re-issued.


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