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

Biochemical Foundations of Human Vital Activity
Biochemistry of Proteins
Chemical Composition and Biological Role of Proteins

Proteins play a crucial role in all vital processes for any Organism. Properties of living organisms such as irritability, contractility, Digestion, and the capacity for growth, reproduction, and movement are all associated with proteins. Consequently, proteins are the primary carriers of life. Compounds similar to proteins do not occur in inanimate nature.

Proteins are high-molecular-weight, nitrogen-containing substances that yield Amino Acids upon Hydrolysis. Sometimes proteins are referred to as proteins (from the Greek proteus — first, primary), thereby defining their paramount role in the vital activity of all organisms. In The Human Body, protein accounts for an average of 45% of dry body mass (12–14 kg). Its content varies across different tissues (Table 18). The highest amount of protein is found in Muscles, bones, Skin, the digestive tract, and other dense tissues.

The daily protein requirement for an adult human who does not engage in sports averages 1.3 g per 1 kg of body mass, or about 80 g. During high Energy Expenditure, this requirement increases by approximately 10 g for every 2100 kJ of increased energy output.

Proteins enter the body predominantly through animal-derived foods. Plants contain significantly less protein: vegetables and fruits contain a mere 0.3–2.0% of fresh tissue mass; the highest protein content is found in legumes (20–30%), grains (10–13%), and mushrooms (3–6%).

Elementary composition of proteins. The most important Chemical elements of all proteins are carbon (50–55%), oxygen (21–23%), hydrogen (6.5–7.3%), nitrogen (15–18%), and sulfur (0.3–2.5%). Phosphorus, iron, iodine, copper, manganese, and other chemical elements have also been detected within proteins.

Class="center">TABLE 18 Protein content in human Organs and tissues

The quantitative and qualitative composition of individual proteins varies. A general idea of The complexity of protein chemical composition is provided by the molecular formula of Hemoglobin, the gas-transporting protein in the body: C₃₀₃₂H₄₈₁₆O₈₇₂N₇₈₀S₈Fe₄.

All proteins contain a constant amount of nitrogen, averaging 16%. Therefore, protein intake by the body is determined based on The amount of ingested nitrogen (the dietary nitrogen percentage is multiplied by a conversion factor of 6.25).

Amino Acid Composition. Proteins consist of amino acids. About 200 different Amino acids are known, yet only 20 are used to build proteins in animal and plant tissues. These amino acids are referred to as standard (or primary).

Along with standard amino acids, certain proteins contain other, non-standard amino acids. Each such amino acid is derived from one of the 20 standard amino acids. For example, 4-hydroxyproline and 5-dihydroxylysine are derivatives of Proline and Lysine and form part of Collagen, a Connective Tissue protein:

The Muscle contractile protein Myosin contains the non-standard amino acid N-methyllysine:

Simple and Conjugated Proteins. Depending on their chemical composition, proteins are divided into simple and conjugated. Simple proteins consist solely of amino acids, which include Water-soluble ones (Histones, albumins, fibrinogen) and insoluble ones (globulins, myosin, collagen, ossein, keratin). Conjugated proteins consist of protein and non-protein moieties. The non-protein moiety may be represented by CARBOHYDRATES, Nucleic Acids, Lipids, phosphoric acid, or pigmented (chromo-) substances. Depending on The Nature of the non-protein part, conjugated proteins are classified into Glycoproteins, Nucleoproteins, Lipoproteins, Phosphoproteins, and Chromoproteins. All of them perform diverse Functions in the body.

BIOLOGICAL FUNCTIONS OF proteins

✵ Structural (plastic). In combination with lipids, proteins form The Structure of all Cell membranes and The basis of Cell Cytoplasm. The structural framework of connective tissue is formed by proteins such as collagen (found in Cartilage and tendons), keratin (found in skin), and Elastin (found in ligaments and Blood vessel walls).

✵ Catalytic. This function is performed by specific Enzymes (see Chapter 6), which regulate METABOLISM and Energy Exchange in the body. If enzymes fail to function within a cell, biochemical reactions cease, and the living cell may perish.

✵ Contractile. All types of contraction and movement in skeletal muscles, the myocardium, and other contractile tissues are driven by the contractile proteins Actin and myosin.

✵ Transport. Proteins are capable of binding and transporting individual molecules and ions via the bloodstream or across cell membranes. For instance, blood erythrocyte hemoglobin carries oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs; muscle Myoglobin transports oxygen within Muscle tissue to sites of utilization. Specific blood proteins transport Fatty acids, lipids, iron, and certain Hormones.

✵ Protective. Proteins of The Immune System, gamma globulins, "recognize" and bind foreign substances entering the body, thereby defending it against Viruses, Bacteria, and Cells of other organisms. The protein interferon also performs a protective function. Blood Plasma Proteins fibrinogen and Thrombin participate in blood clotting processes, preventing blood loss during injuries.

✵ Hormonal, or regulatory. Highly specific protein hormones regulate metabolism (see Chapter 8).

✵ Receptor. Many proteins act as receptors for hormones, Neurotransmitters, and other BIOLOGICALLY ACTIVE SUBSTANCES. They carry out selective recognition, binding, and transmission of their regulatory effects.

✵ Transmission of hereditary information. Proteins are constituents of Chromosomes and participate in the Replication of Genetic information and The regulation of GROWTH AND REPRODUCTION.

✵ Supporting. The elasticity and strength of skeletal bones, skin, and tendons are primarily maintained by the proteins Collagen and elastin.

✵ Energy function. Approximately 10–15% of the body's Energy Requirements are met by proteins. The oxidation of 1 g of protein releases 17 kJ (4.1 kcal) of energy.