Functional Groups

• Aldoses contain the aldehyde group - Monosaccharides in this group are glucose, galactose, ribose, and glyceraldehyde.
• Ketoses contain the ketone group - The major sugar in this group is fructose.
• Reducing: Contain a hemiacetal or hemiketal group. Sugars include, glucose, galactose, fructose, maltose, lactose
• Non-reducing: Contain no hemiacetal groups. Sucrose and all polysaccharides are in this group.

As you know, carbohydrates or glucids are polyhydroxylated aldehydes or ketones, their derivatives and polymers. Glucose is a typical example of carbohydrates.

Observe that glucose has an aldehyde group (drawed in red) and five hydroxyl groups.

Most of carbohydrates are present with a cyclic structure in nature, as a consequence of internal linkages between the carbonyl carbon (of the aldehyde or ketone group) with one of the hydroxyl groups in the same molecule.

Considering the polymerization degree (PD) of carbohydrates, they can be classified in Monosaccharides, Oligosaccharides and Polysaccharides. Monosaccharides:

Monosaccharides are formed by a single molecule. It means that when hydrolyzed they can not release simpler molecules. Examples of this group of carbohydrates are glucose, ribose and fructose, among others.

Monosaccharides can be subclassified according to different criteria, for example:

According to the main (carbonyl) function:

If the carbonyl group belongs to an aldehyde function, the monosaccharide is classified as an aldose. Glucose is a typical aldose. If the main function is a ketone, then the monosaccharide is classified as a ketose.

Fructose (see structure below) is a ketose, since it is a polyhydroxylated ketone.

According to the number of carbons:

Monosaccharides can be classified in trioses, tetroses, pentoses, hexoses, heptoses and octoses, according to the number of carbons in the molecule.

According to the steric series:

According to the type of esteroisomers, monosaccharides can be classified as L or D (most of the carbohydrates in the animal kingdom belongs to D series)

According to the kind or anomer:

According to the position of the anomeric hydroxyl, monosaccharides can be classified as Alpha or Beta. Usually, these criteria are combined for describing a monosaccharide, e.g. a compound can be described as a Beta-D-aldohexose.

Oligosaccharides:

They are formed by 2-9 monomers linked through glycosidic linkages; in other words, when hydrolyzed these compounds release 2 to 9 monosaccharides (some texts say up to 20; in fact, oligosaccharides release “a few” monosaccharides).

According to the number of monosaccharides in the oligosaccharide, oligosaccharides can be dissacharides, trisaccharides, tetrasaccharides, etc. Disaccharides, formed by just 2 monosaccharides, are the most important subgroup of oligosaccharides. Disaccharides that appear in nature are lactose, or milk sugar (formed by galactose and glucose), and sucrose or table sugar (formed by fructose and glucose). Other important disaccharides are produced as result of starch digestion: maltose and isomaltose. These disaccharides are, both of them, formed by two molecules of glucose, but linked in different ways. Cellobiose is a third dissacharide formed also by two molecules of glucose, but linked in such a way that animals can not break, unless animals have in the digestive system specific microorganisms that hydrolyze these linkages, as herbivors have (Cellobiose is formed as result of the digestion of cellulose).

Polysaccharides:

Polysaccharides are carbohydrates formed by more than 9 monosaccharides (some texts say more than 10 monosaccharides, other texts say more than 20…in fact, they usually are formed by a lot of monosaccharides!). When the polysaccharides are formed by the same type of monosaccharides, they are called homopolysaccharides.

34.Amino acids are biologically important organic compounds composed of amine (-NH₂) and carboxylic acid (-COOH) functional groups, along with a side-chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, though other elements are found in the side-chains of certain amino acids.

Protein structure is the biomolecular structure of a protein molecule. Each protein is a polymer – specifically a polypeptide – that is a sequence formed from various L-α -amino acids (also referred to as residues). Secondary structure refers to highly regular local sub-structures. Two main types of secondary structure, the alpha helix and the beta strand or beta sheets, were suggested in 1951 by Linus Pauling and coworkers. These secondary structures are defined by patterns of hydrogen bonds between the main-chain peptide groups. They have a regular geometry, being constrained to specific values of the dihedral angles ψ and φ on the Ramachandran plot. Both the alpha helix and the beta-sheet represent a way of saturating all the hydrogen bond donors and acceptors in the peptide backbone. Some parts of the protein are ordered but do not form any regular structures. They should not be confused with random coil, an unfolded polypeptide chain lacking any fixed three-dimensional structure. Several sequential secondary structures may form a " supersecondary unit". Tertiary structure refers to three-dimensional structure of a single, double, or triple bonded protein molecule. The alpha-helixes and beta pleated-sheets are folded into a compact globular structure. The folding is driven by the non-specific hydrophobic interactions (the burial of hydrophobic residues from water), but the structure is stable only when the parts of a protein domain are locked into place by specific tertiary interactions, such as salt bridges, hydrogen bonds, and the tight packing of side chains and disulfide bonds. The disulfide bonds are extremely rare in cytosolic proteins, since the cytosol is generally a reducing environment. Quaternary structure is the three-dimensional structure of a multi-subunit protein and how the subunits fit together. In this context, the quaternary structure is stabilized by the same non-covalent interactions and disulfide bonds as the tertiary structure. Complexes of two or more polypeptides (i.e. multiple subunits) are called multimers. Specifically it would be called a dimer if it contains two subunits, a trimer if it contains three subunits, and a tetramer if it contains four subunits. The subunits are frequently related to one another by symmetry operations, such as a 2-fold axis in a dimer. Multimers made up of identical subunits are referred to with a prefix of " homo-" (e.g. a homotetramer) and those made up of different subunits are referred to with a prefix of " hetero-" (e.g. a heterotetramer, such as the two alpha and two beta chains of hemoglobin).

35.vitamins. A vitamin is an organic compound and a vital nutrient that an organism requires in limited amounts. An organic chemical compound is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and must be obtained through the diet; thus, the term " vitamin" is conditional upon the circumstances and the particular organism. For example, ascorbic acid (vitamin C) is a vitamin for humans, but not for most other animal organisms. Supplementation is important for the treatment of certain health problems, but there is little evidence of nutritional benefit when used by otherwise healthy people. Vitamins have diverse biochemical functions. Some, such as vitamin D, have hormone-like functions as regulators of mineral metabolism, or regulators of cell and tissue growth and differentiation (such as some forms of vitamin A). Others function as antioxidants (e.g., vitamin E and sometimes vitamin C). The largest number of vitamins, the B complex vitamins, function as precursors for enzyme cofactors, that help enzymes in their work as catalysts in metabolism. In this role, vitamins may be tightly bound to enzymes as part of prosthetic groups: For example, biotin is part of enzymes involved in making fatty acids. They may also be less tightly bound to enzyme catalysts as coenzymes, detachable molecules that function to carry chemical groups or electrons between molecules. For example, folic acid may carry methyl, formyl, and methylene groups in the cell. Although these roles in assisting enzyme-substrate reactions are vitamins' best-known function, the other vitamin functions are equally important.

Vitamins are classified as either water-soluble or fat-soluble. In humans there are 13 vitamins: 4 fat-soluble (A, D, E, and K) and 9 water-soluble (8 B vitamins and vitamin C). Water-soluble vitamins dissolve easily in water and, in general, are readily excreted from the body, to the degree that urinary output is a strong predictor of vitamin consumption. Because they are not as readily stored, more consistent intake is important. Many types of water-soluble vitamins are synthesized by bacteria. Fat-soluble vitamins are absorbed through the intestinal tract with the help of lipids (fats). Because they are more likely to accumulate in the body, they are more likely to lead to hypervitaminosis than are water-soluble vitamins. Fat-soluble vitamin regulation is of particular significance in cystic fibrosis.