BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 5. STRUCTURE, PROPERTIES, AND FUNCTIONS OF PROTEINS

5.1. Structure and properties of amino acids making up proteins. Peptide bonds

5.1.4. The peptide bond. Structure and biological properties of peptides

α-Amino Acids can be covalently linked to each other via peptide bonds. Such a bond is formed between the α-carboxyl group of one Amino Acid and the α-amino group of another, thus representing an amide bond. As a result, a Water molecule is eliminated:

Class="center">

Structure of a peptide. The number of amino acids in a peptide can vary significantly. Peptides containing up to 10 Amino acids are called oligopeptides. The name of such molecules often indicates the number of amino acid residues they contain: tripeptide, pentapeptide, octapeptide, and so on.

Peptides containing more than 10 amino acids are called Polypeptides, while those consisting of more than 50 amino acid residues are generally referred to as Proteins. However, these terms are somewhat arbitrary, as the term "protein" is frequently used in literature to designate polypeptides containing fewer than 50 amino acid residues. For instance, the hormone Glucagon, which consists of 29 amino acids, is referred to as a protein hormone.

The amino acid monomers that make up proteins are called amino acid residues. The amino acid residue with a free amino group is termed the N-terminus and is written on the left, whereas the one with a free α-carboxyl group is termed the C-terminus and is written on the right. Peptides are written and read starting from the N-terminus. The linkage of repeating atoms in the polypeptide chain -NH-CH-CO- is called the peptide backbone:

To name a polypeptide, the suffix -yl is added to the abbreviated names of the amino acid residues, except for the C-terminal amino acid. Thus, the tetrapeptide Ser-Gly-Pro-Ala is read as seryl-glycyl-prolyl-Alanine:

The peptide bond formed by the imino group of Proline differs from other peptide bonds because the nitrogen atom of the peptide group is bonded not to hydrogen, but to a radical.

Peptides differ in their Amino Acid Composition, number, and sequence of amino acids. Ser-His-Pro-Ala and Ala-Pro-His-Ser are two distinct peptides, despite having identical quantitative and qualitative amino acid compositions.

CHARACTERISTICS OF THE peptide bond. The peptide bond exhibits partial double-bond character; therefore, it is shorter than other bonds in the peptide backbone, which makes it less flexible. The electronic STRUCTURE OF THE peptide bond determines the planar, rigid structure of the peptide group. The planes of the peptide groups are angled relative to one another (Fig. 5.1).

The bond between the α-carbon atom and the α-amino or α-carboxyl group is capable of free rotation (although constrained by the size and Nature of the side chains), which allows the polypeptide chain to adopt various Conformations.

Fig. 5.1. Spatial orientation of peptide group planes and α-carbon atoms

Peptide bonds typically exist in the *trans* conformation, meaning that the α-carbon atoms are located on opposite sides of the peptide bond. Consequently, the amino acid side chains are situated at the maximum possible distance from each other in space (Fig. 5.2).

Fig. 5.2. *Trans* conformation of peptide bonds.

The functional groups -CO- and -NH- that form peptide bonds are uncharged yet polar, and can participate in Hydrogen bond formation.

Peptide bonds are extremely stable and do not spontaneously break under normal physiological conditions within The Cell (neutral pH, body Temperature). In laboratory practice, the Hydrolysis of peptide bonds in proteins is carried out in a sealed ampoule with concentrated (6 mol/L) Hydrochloric acid at temperatures exceeding 105 °C. Complete Protein Hydrolysis into free amino acids takes approximately 24 hours.

In living organisms, peptide bonds in proteins are cleaved by Proteolytic Enzymes (from the English *protein* - protein, *lysis* - destruction), known as proteases or peptidases.

To detect PROTEINS AND PEPTIDES in solution, as well as for their quantitative determination, the biuret test is used (yielding a positive result for substances containing at least two peptide bonds).

Biological Role of peptides. A wide variety of peptides are produced in The Human Body, which participate in The regulation of various biological processes and exhibit high physiological activity.

The number of amino acid residues in The structure of biologically active peptides can range from 3 to 50. Among the "smallest" peptides are thyrotropin-releasing hormone and Glutathione (tripeptides), as well as enkephalins, which consist of 5 amino acids. However, the majority of biologically active peptides contain more than 10 amino acids. For example, neuropeptide Y (an appetite regulator) contains 36 amino acids, and corticoliberin contains 41. Some peptides, including most Peptide Hormones, contain peptide bonds formed by the α-amino group and α-carboxyl group of adjacent amino acids. As a rule, they are synthesized from inactive protein precursors, in which specific proteolytic enzymes cleave specific peptide bonds.

Angiotensin II is an octapeptide formed from the Blood Plasma protein angiotensinogen through the sequential action of two proteolytic enzymes. The first proteolytic enzyme, renin, cleaves a 10-amino-acid peptide from the N-terminus of angiotensinogen, known as angiotensin I. The second proteolytic enzyme, carboxydipeptidyl peptidase, cleaves Two amino acids from the C-terminus of angiotensin I, resulting in The formation of biologically active angiotensin II, which participates in the Regulation of Blood pressure (BP) and water-salt METABOLISM in the Body:

Certain biologically active peptides may contain either unusual amino acids or unusual bonds between amino acids that are not found in proteins. An example of a peptide containing an Unusual amino acid linkage is the tripeptide glutathione, which is built from Cysteine, glutamate, and Glycine:

The N-terminal amino acid glutamate is linked to the second amino acid, cysteine, not through its α-carboxyl group, but through the γ-carboxyl group of its side chain. Glutathione is a widespread peptide in the human body. It can be used in oxidation-reduction reactions as a hydrogen donor and acceptor and is essential for the functioning of A number of enzymes.

The Functions of peptides depend on their Primary Structure. Angiotensin I is very similar in structure to angiotensin II (possessing only two additional amino acids at the C-terminus), yet it is biologically inactive.

Alterations in the amino acid composition of peptides often lead to the loss of certain biological properties and the acquisition of new ones. As an example, we can examine the Structure and properties of two peptide hormones: oxytocin and vasopressin. In the Hypothalamus, oxytocin and vasopressin are produced as a result of partial (limited) proteolysis of protein precursors. From the hypothalamus, these hormones travel along nerve fibers within secretory granules to the nerve terminals of axons located in the posterior Pituitary Gland. Upon specific stimuli, these hormones are released into the bloodstream:

Oxytocin and vasopressin share many structural features:

✵ both contain nine amino acid residues;

✵ seven out of the nine amino acid residues are identical;

✵ two cysteine residues are linked by a disulfide bond;

✵ at the C-terminus of the peptides, the α-carboxyl group of glutamate is amidated.

Despite minor differences in Amino Acid Sequence (Amino Acid Substitutions at positions 3 and 8), these hormones differ significantly in their physiological action. Thus, oxytocin is released into the blood during breastfeeding, causing the contraction of myoepithelial Cells in the ducts of the Mammary Glands and stimulating milk ejection. In addition, oxytocin affects the smooth musculature of the Uterus during childbirth, inducing its contraction.

Unlike oxytocin, the main physiological action of vasopressin is to increase water reabsorption in the Kidneys when blood pressure or blood volume decreases (hence the second name of this hormone is antidiuretic hormone). Furthermore, vasopressin causes the constriction of vascular smooth Muscle cells (SMCs).

It is worth noting that the presence of a basic amino acid at position 8 is crucial for expressing antidiuretic activity, while an amino acid with a hydrophobic side chain at position 3 is essential for SMC contraction.

Since peptides are potent regulators of biological processes, they can be considered as medicinal agents. The main obstacle to the THERAPEUTIC USE OF peptides is their rapid degradation in the body.

One of the most important research outcomes is not only The Study of peptide structures but also The production of synthetic analogs of Natural peptides with targeted modifications in their STRUCTURE AND FUNCTIONS. For example, the peptide 1-deamino-8-D-Arginine vasopressin (DDAVP) has been synthesized, the structure of which is shown in the diagram:

In the structure of this peptide (compared to vasopressin), there is no amino group at the N-terminus, and D-arginine is present at position 8 instead of L-arginine. Such a synthetic peptide possesses exclusively antidiuretic activity and is chemically stable, meaning that upon administration, it induces a prolonged response. Compared to the natural hormone, this artificial analog is more effective in treating hormone deficiencies.

According to their primary physiological action, peptides can be divided into:

✵ hormonally active ones (oxytocin, vasopressin, hypothalamic releasing hormones, melanocyte-stimulating hormone, glucagon, etc.);

✵ those regulating digestive processes (gastrin, cholecystokinin, vasoactive intestinal peptide, gastric inhibitory peptide, etc.);

✵ that regulate vascular tone and blood pressure (bradykinin, kallidin, angiotensin II);

✵ that regulate appetite (leptin, neuropeptide Y, melanocyte-stimulating hormone, β-endorphins);

✵ that possess analgesic properties (enkephalins, endorphins, and other opioid peptides). The pain-relieving effect of these peptides is hundreds of times greater than the analgesic effect of morphine;

✵ that participate in the regulation of Higher Nervous Activity and biochemical processes associated with the mechanisms of Sleep, learning, memory, the onset of fear, etc.

However, this Classification of peptides is quite conventional. Evidence has emerged indicating that many peptides exhibit a broad spectrum of action. For instance, melanocyte-stimulating hormone, In addition to stimulating pigmentation, is involved in appetite regulation (along with leptin, it suppresses the sensation of hunger and acts as an antagonist to neuropeptide Y). At the same time, β-endorphins, besides their analgesic effect, act as synergists to neuropeptide Y, thereby enhancing the feeling of hunger. Furthermore, the aforementioned vasopressin, apart from its antidiuretic and vasoconstrictive effects, has been shown to improve memory.