Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Introduction to Biochemistry
Chemical Structure of the Human Body

The Human Body is composed of organic and inorganic substances (Fig. 4). Water accounts for approximately 60 % of body mass, while minerals make up an average of 4 %. Organic matter is primarily represented by Proteins (18 %), Lipids (15 %), and CARBOHYDRATES (2—3 %). All substances in the body, much like inanimate nature, are built from atoms of various chemical elements.

Chemical elements of the Body

Out of 110 known chemical elements, the human body is primarily composed of 24 (Table 1). Depending on their concentration in the body, chemical elements are divided into major elements, Macronutrients, micronutrients, and trace (ultramicronutrient) elements. The major elements include oxygen (65—70 %), carbon (15—18 %), hydrogen (8—10 %), and nitrogen (2—3 %), which together account for roughly 98 % of total body mass. Macronutrients are elements whose concentration in the body is at least 0.1 % of body mass (Ca, P, S, K, Cl, Na, Mg). Together, they account for 99.9 % of body mass. Micronutrients are elements with a concentration of at least 0.001 % of total body mass (Cu, Si, Mn, Co, etc.). If the concentration of a chemical element in the body is less than 0.001 % of body mass, it is classified as an ultramicronutrient.

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Fig. 4 Relative Chemical composition of the human body

TABLE 1 Chemical elements that make up the human body

The chemical elements used to form the body's substances possess the following properties:

✵ their atoms are small in size, allowing them to form compact molecules capable of penetrating Cell membranes;

✵ they readily enter into chemical interactions, forming strong covalent bonds within molecular structures;

✵ their compounds are highly water-soluble and easily assimilated by the Organism;

✵ certain elements (P, S, N) can form labile, energy-rich chemical bonds and participate in biochemical reactions associated with energy storage and release;

✵ the ability of the carbon atom to form carbon-carbon bonds enables the rapid conversion of various Organic compounds within the body.

Specific chemical elements accumulate unevenly across different Organs and Tissues. For instance, Bone tissue accumulates Calcium and phosphorus, Blood stores iron, The Thyroid Gland concentrates iodine, the Liver holds copper, and the Skin accumulates strontium, etc. The quantitative and qualitative COMPOSITION OF THE body's chemical elements depends both on external environmental factors (such as diet and ecology) and on the specific Functions of individual organs.

Organic Substances

In the human body, the majority of chemical substances are represented by diverse organic compounds. Organic substances are carbon-based compounds in which carbon atoms are bonded to one another and to other atoms (H, O, N, P, S) or groups of atoms.

The body contains both low-molecular-weight (simple) and high-molecular-weight (complex) organic substances. The diverse high-molecular-weight compounds of the body and its cellular structures are built from a small number of simple low-molecular-weight organic compounds that either enter the body from the external environment or are synthesized within it. Such compounds include Amino Acids, nitrogenous bases, glucose, and Fatty acids. These substances also perform many independent functions within the body. The simple molecules utilized to form high-molecular-weight compounds include:

The primary high-molecular-weight compounds in the human body are macromolecules of proteins and Nucleic Acids, as well as Introduction/36.html">Carbohydrates and lipids. They possess a specific chemical Structure that determines their properties and biological role in the organism.

Structure of Organic Compounds. In all organic compounds, the carbon atom (C) exhibits a valence of 4. It is capable of forming paired orbitals with four electrons from other atoms—most commonly carbon, hydrogen, or oxygen. This results in The formation of four electron pairs shared by both atoms, as illustrated by the electronic formula of acetic acid:

In structural formulas, the shared electron pair forming a chemical bond is represented by a dash.

The unique electronic STRUCTURE OF THE carbon atom enables it to form an endless variety of carbon-carbon bonds:

This gives rise to linear, branched, and closed (cyclic) carbon chains that serve as the backbone of organic compounds. When carbon atoms utilize a single valence unit in their interaction with one another while all remaining valences are satisfied by other atoms, such compounds are termed saturated. Carbon atoms can also form double and triple bonds between themselves. Such compounds are called unsaturated:

Many organic acids found in the human body have a linear carbon Skeleton structure, such as the fatty palmitic acid (С₁₆Н₃₂О₂):

Certain amino acids, as well as Vitamins and some other substances, feature a branched structure, for instance, The amino acid valine:

Organic substances with a linear or branched carbon chain are called acyclic, whereas those with a closed carbon chain are termed cyclic.

Cyclic, or closed, carbon chains may contain varying numbers of carbon atoms, can be saturated or unsaturated, and may also include atoms of other chemical elements. Carbocyclic organic compounds are those whose rings contain only carbon atoms. If other atoms—such as N, S, or O—are incorporated into the carbon skeleton, such substances are referred to as heterocyclic.

Many carbocyclic compounds incorporate the rings of the following substances:

The heterocyclic compounds found in the body, as well as in Antibiotics, vitamins, and certain pharmacological agents, contain pyrrole, pyrimidine, purine, and thiophene rings:

Pyrrole is a component of Hemoglobin and vitamin B₁₂, pyrimidine and purine are Components of nucleic acids and ATP, and thiophene is a component of vitamin H.

Carbon atoms in organic compounds are capable of interacting with specific groups of atoms known as functional groups.

Functional groups are specific atomic groupings whose presence in molecules imparts chemical properties characteristic of a given class of substances. The main functional groups and their corresponding classes of organic compounds are presented in Table 2. The presence of specific functional groups in substances influences their biological activity and the execution of particular functions.

The properties of organic substances and their ability to participate in exchange reactions depend on the type of chemical bonding between atoms in the molecule.

Types of chemical bonds. The bond between atoms in a molecule, or the type of chemical bond in a substance, influences its properties and biological role.

In organic molecules, the predominant type of chemical bond is covalent. Ionic, hydrogen, and Other types of chemical bonds are also possible.

A covalent bond is formed between atoms through the sharing of electron pairs. This is the strongest type of chemical bond. Breaking a covalent bond requires an energy input of 83 kcal ∙ mol^-1. Therefore, the carbon skeletons of organic compounds are relatively stable in the absence of energy supply. In structural formulas, a covalent bond is denoted by a dash (-).

TABLE 2. Main functional groups and classes of organic compounds

Certain organic substances contain a weaker covalent bond, the Cleavage of which releases Free energy (approximately 7 kcal). Such a bond is conventionally termed high-energy (macroergic) and is typical of substances that perform an energy function in living Cells, such as ATP, ADP, creatine phosphate, etc. A high-energy bond in compounds is denoted by a tilde (~). An ATP molecule contains two such bonds: adenosine — P—P —P.

An ionic bond is formed between oppositely charged ions through electrostatic attraction. This type of bond is characteristic, for example, of sodium chloride (table salt):

In an aqueous environment, substances with an ionic bond readily dissociate into positively charged ions (cations) and negatively charged ions (anions). Living organisms contain a relatively small number of compounds with ionic bonds.

A Hydrogen bond is formed through the electrostatic attraction between a positively charged hydrogen atom and negatively charged atoms of oxygen, nitrogen, and other elements. This bond is represented by a dashed line:

Hydrogen Bonds can be either intermolecular or intramolecular. They drive the aggregation of molecules into larger structures, such as water molecules (see Chapter 4). Additionally, they play a crucial role in the formation and stabilization of the spatial structures of proteins and nucleic acids. Hydrogen bonds are relatively weak, with a dissociation energy of approximately 5 kcal ∙ mol^-1, allowing them to break easily. This disruption leads to the dissociation of compounds or Changes in the conformation (structure) of macromolecules. Conformational Changes in organic macromolecules underpin their biological functions; for instance, Muscle contraction occurs precisely due to the conformational changes of contractile proteins.

Formulas of Organic Compounds. In chemistry, all substances are represented by molecular (empirical) formulas that reflect their Qualitative and quantitative composition.

Biological chemistry primarily utilizes structural formulas, which illustrate the arrangement and bonding order of atoms and functional groups within a molecule. For instance, glucose (С₆Н₁₂О₆) can be depicted using linear or cyclic structural formulas:

The molecules of many organic compounds can be viewed as a combination of a radical and a functional group. A radical (R) is a molecular residue that is transferred into other molecules during chemical transformations without altering the bonding order of its atoms. For example, in fatty acid molecules, various radicals are attached to a carboxyl group (-СООН), which can undergo changes while leaving the radical intact:

Consequently, the general formulas of organic compounds are frequently written as a combination of a radical and a functional group. For example, the general formula of amino acids is represented as:

Isomerism of Organic Compounds. Organic compounds are characterized by the existence of multiple substances that share the exact same Chemical Composition and molecular weight but differ in molecular structure or the bonding order of their atoms. Such substances are called isomers and exhibit distinct chemical properties. There are two primary types of isomerism: structural isomerism and stereoisomerism.

Structural isomerism arises from differences in the bonding sequence of atoms within an organic molecule. For example, glucose and fructose molecules differ in THE POSITION OF their functional groups along the carbon chain, serving as a classic example of positional isomerism:

Stereoisomerism is characteristic of organic molecules that contain an asymmetric carbon atom (*) bonded to four different atoms or atomic groups arranged differently in space. Stereoisomers are optically active and capable of polarizing a beam of light. They are classified into right-handed (D-isomers) and left-handed (L-isomers) forms, which differ in the spatial orientation of the hydroxyl group (OH) relative to the asymmetric carbon atom:

The phenomenon of isomerism is of immense biological significance, as carbohydrates, amino acids, and proteins are optically active substances that are selectively utilized by the living organism. For instance, Protein Synthesis exclusively utilizes L-isomers of amino acids, whereas D-isomers are occasionally found in malignant tumors. The primary carbohydrates—glucose and fructose—occur in the body as D-isomers.

Classification of Organic Compounds. The vast diversity of organic compounds necessitates a well-structured classification system. Several classification frameworks exist, depending on the underlying principle. Organic substances are typically classified based on their carbon chain structure, the presence of functional groups, and the biological functions they perform in the body.

Based on The structure of the carbon chain, organic compounds are divided into two main classes and several subclasses:

Based on the presence of functional groups that confer specific chemical properties, organic substances are grouped into distinct classes (see Table 2).

In biochemistry, organic compounds are commonly classified According to the biological functions they perform within the organism. There are four principal classes of organic substances: carbohydrates, lipids, proteins, and nucleic acids.

The transformation of these substances within the body and their role in muscular activity are discussed further below. Their metabolism also involves vitamins, hormones, enzymes, NUCLEOTIDES, phosphate-containing compounds, and other substances.

Inorganic Substances

Inorganic substances in the body include water and minerals. Although water is not a nutrient per se, it plays a vital role in maintaining high physical performance and work capacity in the organism.

Minerals account for 4–10% of body weight. They occur in the body either as free cations and anions (Са²⁺, Mg²⁺, Na⁺, К⁺, Сl^-) or in bound forms as constituents of organic and Inorganic Compounds. The physiological roles of water and minerals, as well as the specifics of their metabolism during muscular activity, are discussed in Chapter 4.

Certain cells in the body produce inorganic substances such as Hydrochloric acid (HCl in Stomach cells), ammonia (NH₃), carbon dioxide (CO₂), and others. Hydrochloric acid is essential for Protein Digestion processes, while ammonia and carbon dioxide are End products of Metabolism. They are either excreted from the body or utilized in The biosynthesis of specific organic compounds (urea, amino acids).