BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 1. STRUCTURE AND REACTIVITY OF BIOORGANIC COMPOUNDS

1.3. Selected Classes of Bioorganic Compounds and Their Biological Significance

1.3.5. Heterofunctional Compounds

Heterofunctional compounds are substances containing different functional groups. They comprise the majority of natural compounds and Biomolecules, as well as a significant number of pharmaceutical agents. The general formula for heterofunctional compounds is X-R-Y, where X represents an NН₂ group (in Amino Acids), an OH group (in hydroxy acids), or a CO group (in oxo (keto) acids). In all of these compounds, the Y group is a carboxyl group. Another important group of heterofunctional compounds consists of amino alcohols and aminophenols, in which the X and Y groups are OH and NН₂ groups, respectively.

Hydroxy acids are derivatives of carboxylic acids, characterized by the simultaneous presence of OH and COOH groups in their Structure. They are classified into aliphatic and aromatic hydroxy acids (phenolic acids were discussed above). The number of COOH groups determines their basicity, while the number of OH groups determines their atomic function (polyhydric nature). Isomers of aliphatic hydroxy acids are determined by the relative positions of the COOH and OH groups: α, β, γ, δ. Hydroxy acids serve as metabolic intermediates and are produced in the Organism during the METABOLISM of CARBOHYDRATES, amino acids, and Fatty acids. The most important among them include malic, citric, isocitric, and other acids. This group of substances exhibits properties determined by both the carboxyl and the hydroxyl groups. Through their carboxyl group, hydroxy acids form salts, amides, halogen anhydrides, and esters:

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Through their OH group, hydroxy acids form alcoholates, ethers, and esters. Oxidation of the hydroxyl group yields oxo carboxylic acids. The oxidation of hydroxy acids in biochemical systems also proceeds via dehydrogenation by dehydrogenases, involving a hydrogen atom acceptor—the coenzyme nicotinamide adenine dinucleotide (NAD; Section 7.3). Examples of such reactions include the oxidation of lactic acid to pyruvic acid by oxygen (1) and via dehydrogenation by dehydrogenases (2):

Dehydration of hydroxy acids yields cyclic structures—lactides and lactones—which contain 2 and 1 ester bonds, respectively. The formation of a single lactide molecule requires two molecules of α-hydroxy acids, whereas lactones are "internal anhydrides" of the corresponding γ- and δ-hydroxy acids, meaning they are formed by the Functional groups of a single molecule. Vitamin C serves as an example of a physiologically active lactone:

Oxo carboxylic acids (oxo acids: aldehyde and keto acids) exhibit chemical properties characteristic of both carboxylic acids and aldehydes or ketones. These acids play a leading role in anaerobic Glycolysis (pyruvic acid), act as intermediates in the Krebs cycle (oxaloacetic and α-ketoglutaric acids), and serve as intermediates in the mitochondrial Oxidation of Fatty acids (β-oxidation). One such intermediate in fatty acid β-oxidation (Section 14) is acetoacetic acid, which is converted in Tissues into β-hydroxybutyric acid and acetone:

These three compounds are collectively referred to as Ketone Bodies or acetone bodies. Determining their levels in Blood and urine is an important diagnostic indicator. In severe forms of Diabetes Mellitus, acetoacetic acid and acetone accumulate in Blood Plasma, leading to an increased excretion of these compounds in the urine, a condition known as acetonuria or ketonuria.

Amino alcohols and aminophenols, as hydrocarbon derivatives, contain both NH₂ and OH groups in their structure and exhibit similar properties. As bases, amino alcohols form salts with acids: НО-(СН₂)_n-NН₃⁺СI^-, and upon reacting with Water, they form hydroxides НО-(СН₂)_n-Н₃⁺ОН^-. Important representatives of amino alcohols include Biogenic Amines and their derivatives: НО-СН₂-СН₂-NH₃⁺ (ethanolamine (colamine)), HO-CH₂-CH₂-N³ =(СН₃)₃ (Choline (trimethyl-β-hydroxyethylammonium hydroxide)), and (СН₃)₃= N⁺ -СН₂-СН₂-О-СО-СН₃ (acetylcholine).

Aminophenols are weaker bases than amino alcohols; they react with both acids (via the Basic Properties of the NH₂ group) and bases (via the acidic Properties of the phenolic OH group), thus acting as amphoteric compounds. Physiologically active biogenic amines include the aminophenols dopamine, adrenaline, and noradrenaline, whose effects are mediated through Cell Plasma Membrane Receptors. Synthetic analogues of catecholamines are used as neurotropic therapeutic agents.