Proteins are the primary building material of organisms. Different proteins have different functions. If we think of the body as a building, we can compare each material or component—such as iron, bricks, cement, windows and elevators—to a protein in the organism.
Proteins are mainly composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S). However, phosphorus (P), iron (Fe), copper (Cu) and zinc (Zn) can also be found in the structure of proteins.
The building blocks of proteins are amino acids. Some proteins consist only of amino acids, while others also contain molecules such as carbohydrates and fats.
Amino acids
Although nearly 200 amino acids have been identified in nature, generally 20 of them occur in proteins.
Amino acids are molecules consisting of carbon chains that contain at least one carboxyl group (-COOH) and at least one amino group (-NH2) in their structure (proline has an imino group (-NH)). The number of carbons in the chain usually varies between 2 and 11, depending on the type of amino acid.
Glycine, the simplest amino acid, consists of a carboxyl group and amino group bonded to 1 carbon atom and contains two carbon atoms.
A peptide bond is formed between the carboxyl group of one amino acid and the amino group of the other amino acid. In this way, the structure formed due to the combination of two amino acids is called a dipeptide and the structure formed when three amino acids are connected by peptide bonds is called a tripeptide. In this way, amino acids join through peptide bonds to form peptides.
Proteins are peptides usually made up of 100 or more amino acids. However, some proteins contain fewer than 100 amino acids.
The number and sequence of amino acids in a protein, and hence its chain length, are encoded in the DNA of the organism that produces it. This is why proteins are specific to the organism.
Structure of proteins
The structure in which amino acids combine to form a chain is the proteins’ “primary structure.” However, this structure does not remain in a straight chain; The amino acids in the chain interact with each other and as a result of this interaction, folds occur in the chain.
As a result of these interactions, hydrogen bonds are usually formed between oxygen and nitrogen. A turn generally occurs every 4 amino acids. As a result, the protein gains a helix structure; This structure of proteins is called “secondary structure.” Proteins such as myosin, keratin and epidermin have secondary structures.
The hydrogen bonds that provide the formation of the secondary structure are weak bonds, but the large number of hydrogen bonds in the structure makes the secondary structure stable.
However, if two sulfur-containing amino acids coincide in the folds of the chain, a disulfide bond (S-S) is also established between these two amino acids. The disulfide bond in the secondary structure makes the structure much stronger. In general, proteins show secondary structures in two different ways: β-sheet and α-helix.
The secondary structure formed takes on a more complex structure by showing hydrogen bonds, disulfide bonds, van der Waals bonds and hydrophobic interactions. This structure of proteins is called “tertiary structure.” Globular proteins generally show a tertiary structure.
“Quaternary structure” emerges when more than one polypeptide chain comes together with these interactions, Hemoglobin is a protein with a quaternary structure.
Image: Open Oregon – Protein Structure, CC BY 4.0
Proteins have different molecular weights depending on the number of amino acids they contain. Insulin weighs 6,000 Da while urease weighs 480,000 Da.
Classification of proteins
Proteins can be classified according to different parameters. These classifications are as follows;
I) According to the configuration
II) According to the chemical composition
III) According to the function
I) According to the configuration
1. Fibrous proteins; are rod-shaped proteins that are insoluble in water. Collagen, keratin, elastin and tropomyosin can be given as examples.
2. Globular proteins; are water-soluble round-shaped proteins. Serum albumin, hemoglobin, enzymes, antibodies and hormones can be given as examples.
II) According to the chemical composition
1. Simple proteins; are proteins that release only amino acids and their derivatives when hydrolyzed. Examples are albumins, globulins, glutelins, prolamins, protamines, histones, collagen, keratin, fibrin, myosin, elastin and epidermin.
2. Conjugated proteins; are proteins that release substances other than amino acids and their derivatives when they undergo hydrolysis. Examples include phosphoproteins, lipoproteins, glycoproteins, nucleoproteins, chromoproteins and metalloproteins.
III) According to the function
1. Enzymes; amylase and lactase can be given as examples.
2. Structural proteins; Examples include glycoproteins and lipoproteins.
3. Contractile proteins; myosin, actin and tubulin can be given as examples.
4. Hormones; Examples include insulin and growth hormones.
5. Transport proteins; Examples include hemoglobin, transferrin and serum albumin.
6. Storage proteins; Hordein and gliadin can be given as examples.
7. Protective proteins; Examples include antibodies and immunoglobulins.
8. Toxic proteins; snake venom and ricin are examples.
References
Saldamlı, İ. (Ed.). (2021). Gıda Kimyası. Hacettepe Üniversitesi Yayınları.
Bilişli, A. (2012). Gıda Kimyası. Sidas Medya.
Gökalp, H. Y., Nas, S., & Certel, M. (2002). Biyokimya-I: Temel Yapılar ve Kavramlar. Pamukkale Üniversitesi Mühendislik Fakültesi.
Aksoy, M. (Ed.). (2022). Beslenme Biyokimyası (7. baskı). Ankara Nobel Tıp Kitabevleri.
Alberts, B., Hopkin, K., Johnson, A. D., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2023). Essential Cell Biology (6th ed.). W. W. Norton & Company.
Raymond, J. L., & Morrow, K. (2023). Krause ve Mahan Besin ve Beslenme Bakım Süreci (16. baskı; G. Akbulut, Çev. Ed.). Nobel Tıp Kitabevleri.
Callahan, A., Leonard, H., & Powell, T. (2022). Nutrition: Science and Everyday Application (2nd ed.; Protein Structure). Lane Community College / Open Oregon Educational Resources.
Here’s an article that might interest you: Proteins; Functions in the Body, Quality and Daily Needs
Halil İbrahim Akgül
Halil Ibrahim Akgul, Ph.D., is an Assistant Professor of Food Engineering at Bayburt University, where he works in the Division of Food Sciences within the Faculty of Engineering. He received his B.Sc. in Food Engineering from Akdeniz University in 2010, followed by an M.Sc. and a Ph.D. in Food Engineering from Atatürk University in 2015 and 2020, respectively. His formally recognised academic fields are Food Sciences and Engineering, Food Technologies, Dairy and Dairy Products Technology, and Food Microbiology.
Dr. Akgul has more than fifteen years of professional, research, and academic experience in food science. Between 2011 and 2013, he worked as the food engineer responsible for operations at the Pilot Dairy and Bread Factory of Atatürk University’s Directorate of Agricultural Enterprises. This industrial experience, combined with his subsequent academic research, gave him extensive practical and scientific knowledge of milk processing, dairy production, food quality, hygiene, and product development.
His research is primarily focused on milk and dairy products, dairy microbiology, starter cultures, lactic acid bacteria, exopolysaccharides, butter technology, and the physicochemical, microbiological, sensory, and nutritional properties of foods. His studies have examined the quality and fatty-acid composition of traditional butter, aroma formation in butter produced with different starter cultures, raw-milk quality, the characterisation and antioxidant activity of bacterial exopolysaccharides, traditional dairy products such as çökelek and karın kaymağı, colostrum powder, and the use of fruit-derived ingredients in ice cream production.
His academic record includes peer-reviewed research articles published in national and international journals, including Mljekarstvo, Acta Scientiarum Technology, the Journal of Food Safety and Food Quality, the International Journal of Food Engineering, the Atatürk University Journal of Agricultural Faculty, and ATA Food Journal. Four of the research articles currently listed in his university profile were published in SCI-Expanded-indexed journals. He has also contributed to twelve national and international conference papers and presentations covering dairy products, food microbiology, butter quality, fatty-acid composition, starter cultures, exopolysaccharide-producing bacteria, colostrum, traditional foods, and functional ice-cream formulations.
Dr. Akgul has participated as a researcher or adviser in eleven completed scientific and applied research projects. These projects have addressed the modernisation of pilot dairy-processing facilities; the production and quality characteristics of colostrum powder; the physicochemical and microbiological quality of butter and raw milk; the identification of exopolysaccharide-producing lactic acid bacteria; the development and production optimisation of starter cultures for mould-ripened cheese; the standardisation of traditional dairy products; and the use of sloe berry and persimmon fibre in ice-cream production.
His teaching experience spans undergraduate, associate-degree, and graduate education. The courses he has taught include Food Chemistry, Analytical Chemistry, General Chemistry, Organic Chemistry, Fermented Dairy Products Technology, Butter Production and Processing Technology, Healthy Living and Nutrition, Organic Agriculture and Food, Popular Science, Digital Literacy, and Information Technologies. Through these courses, he combines fundamental food chemistry and engineering principles with dairy technology, nutrition, scientific literacy, and the practical communication of scientific knowledge.
He is the founder and Editor-in-Chief of Gıda ve Sağlık, a multilingual digital publication devoted to food, nutrition, health, food chemistry, food microbiology, and culinary science. Through this platform, he draws upon his academic background, teaching experience, research activity, and practical knowledge of food production to present scientifically grounded information in a detailed and accessible form for an international readership.