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

Biochemical Foundations of Human Vital Activity
Enzymes: Biological Catalysts
Structure of Enzymes and Coenzymes

Currently, about 2,000 Enzymes have been studied in detail. Like Proteins, enzymes feature a complex spatial Tertiary and Quaternary Structure. The native (natural) Structural Organization of enzymes ensures their catalytic function. Disruptions to this structure caused by various factors lead to a loss of enzymatic activity. Simple and complex enzymes. Enzymes are divided into simple and complex ones. Simple enzymes consist solely of protein. Many digestive tract enzymes belong to this group, such as amylase, Pepsin, and Trypsin. Complex enzymes consist of a protein moiety called the apoenzyme and a non-protein moiety called the cofactor. The molecule of a complex enzyme is frequently referred to as a holoenzyme. Cofactors that are loosely bound to the protein component are called Coenzymes. A coenzyme can easily transfer from one enzyme to another. Cofactors that are firmly bound to the protein part are known as prosthetic groups. Cofactors can comprise various organic substances and their complexes, as well as minerals. Many of them are thermostable, yet they can be oxidized by atmospheric oxygen. In The Human Body, A number of cofactors are not synthesized internally but must be obtained through diet. Their structure and involvement in biological processes are discussed below.

The catalytic activity of an enzyme is driven not by its entire molecule, but rather by a small specific region known as the Active Site (Fig. 33). The active site is the part of the enzyme molecule that interacts with the coenzyme and substrate, participating directly in the chemical transformation. The Active Site of Enzymes can be formed by several Functional groups of individual Amino Acids located in different Regions of the protein's polypeptide chain (Fig. 34). Therefore, the native structural organization of an enzyme is crucial for its catalytic activity. If this structure is disrupted, the active site changes, and consequently, so does the enzyme's activity. Certain enzymes consist of multiple protein molecules, meaning they possess a subunit structure. They may feature several active sites or a single unified site formed through the interaction of these subunits.

In addition to the active site, many enzymes possess a regulatory (allosteric) site. This is a specialized region on the enzyme molecule, located away from the active site, where low-molecular-weight modulator substances can bind to alter its structure and activity. Enzymes subject to Allosteric Regulation are frequently positioned at the beginning of metabolic pathways, thereby determining their overall rate. These are referred to Key Enzymes. They include Phosphofructokinase, a key enzyme in the anaerobic pathway of carbohydrate oxidation. Its Allosteric regulators include AMP, ATP, and other substances. Depending on The ratio of these modulators, The activity of phosphofructokinase changes along with The rate of glucose oxidation leading to ATP production.

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Fig. 33 Diagram of Enzyme Structure

Fig. 34 Tertiary Structure of phosphorylase a (showing the localization of the active site, Glycogen-binding sites, allosteric centers, and the phosphorylation site)

Cofactors. The catalytic activity of complex enzymes manifests only in the presence of coenzymes:

The coenzyme takes part in forming the enzyme's active site. Many coenzymes have a complex structure and incorporate Vitamins. Thus, the regulatory effect of vitamins on METABOLISM is mediated through enzymes.

Based on their structural features, coenzymes are divided into two major groups: vitamin-derived and non-vitamin (Table 7).

TABLE 7 Main Enzyme Cofactors

Certain coenzymes are components of high biological value products used in sports practice to enhance energy production mechanisms, accelerate recovery processes, and specifically shape body composition; therefore, we will examine their characteristics in greater detail.

Vitamin-derived coenzymes—such as nicotinamide, flavin, Acetylation coenzyme, thiamine pyrophosphate, pyridoxal, and cobalamin—differ in their STRUCTURE AND Functions.

Nicotinamide coenzymes (NAD and NADP) contain Vitamin PP (nicotinamide), while Flavin Coenzymes (FMN and FAD) contain vitamin B₂ (riboflavin). These serve as cofactors for dehydrogenase enzymes, which catalyze Biological Oxidation processes of nutrients. They act as hydrogen acceptors and carriers:

The structure and action of these coenzymes are discussed in greater detail in Chapter 3.

The acetylation coenzyme (CoA-SH) contains vitamin B₃ (pantothenic acid), along with a nucleotide (ADP) and ß-mercaptoethanol featuring an SH group. This coenzyme plays a crucial role in carbohydrate, lipid, and Protein metabolism. It is a component of enzymes that catalyze The transfer of acetyl groups (CH₃-CO-) during The breakdown of carbohydrates and Fatty acids, as well as the synthesis of fatty acids, Steroids, acetylcholine, and amino acid transformations.

Thiamine pyrophosphate (TPP) contains vitamin B₁ (thiamine). It functions as a coenzyme for enzymes that catalyze the decarboxylation (-CO₂) of pyruvic and other keto acids, regulating carbohydrate Breakdown and Oxidation.

Pyridoxal and cobalamin coenzymes are derivatives of vitamin B₆ (Pyridoxal phosphate) and vitamin B₁₂ (cyanocobalamin), respectively. They are part of enzymes that catalyze The conversion of Amino Acids and nitrogenous bases, and accelerate the synthesis of Nucleic Acids and Proteins.

Biotin (vitamin H) serves as the prosthetic group for the enzyme acetyl-CoA carboxylase, which participates in FATTY ACID Biosynthesis. The protein Avidin, found in chicken eggs, can bind to biotin and inhibit this enzyme.

Ubiquinone coenzymes (coenzymes Q) are derivatives of the Fat-soluble vitamins K and E. They take part in tissue Respiration and energy generation processes.

Non-vitamin cofactors can include NUCLEOTIDES (ATP, GTP, ITP, UDP, CTP), heme-containing compounds, Peptides, and various metals. Nucleotides and Metal Ions help the enzyme or substrate adopt the conformation required for their interaction. Heme serves as the prosthetic group for Cytochromes (Components of the Respiratory Chain), catalase, and Other Enzymes.