BIOLOGY Volume 1 - A Guide to General Biology - 2004

3. CHEMICAL COMPONENTS OF LIVING ORGANISMS

3.5. Proteins

3.5.2. Classification of proteins

The complex Structure of protein molecules and the extreme diversity of their Functions make it extremely difficult to establish a single, clear-cut Classification of Proteins based on a single criterion. Tables 3.7–3.9 present three different protein classifications based on various characteristics.

Class="center">Table 3.7. Classification of proteins by structure

Protein class	Characteristics	Function
Fibrous	Secondary structure is most important (tertiary structure is poorly developed or absent) Insoluble in Water Examine high mechanical strength Long parallel polypeptide chains held together by cross-links, forming long fibers or layered structures	Perform structural functions in Cells and organisms. This group includes, for example, Collagen (tendons, bones, Connective Tissue), Myosin (Muscles), Fibroin (silk, spider silk), and keratin (Hair, horns, Nails, feathers)
Globular	Tertiary structure is most important. Polypeptide chains are folded into compact globules Soluble — readily form colloidal Suspensions	Function as Enzymes, Antibodies, and in some cases Hormones (e.g., Insulin), as well as performing several other vital functions
Intermediate	Fibrous in nature, but soluble	An example is fibrinogen, which is converted into insoluble fibrin during Blood clotting

Table 3.8. Classification of proteins by composition


Conjugated Proteins
Name	Prosthetic group	Example
Phosphoproteins	Phosphoric acid	Milk casein Egg yolk vitellin
Glycoproteins	Carbohydrate	Membrane components. Mucin (a saliva component)
Nucleoproteins	Nucleic acid	Viral components Chromosomes Ribosomes
Chromoproteins	Pigment	Hemoglobin — heme (an iron-containing pigment) Phytochrome (a plant-derived pigment) Cytochrome (a respiratory pigment)
Lipoproteins	Lipid	Membrane components Blood lipoproteins — transport form of Lipids
Flavoproteins	FAD (flavin adenine dinucleotide; see sec. 9.3.5)	Component of the Respiratory Electron Transport chain
Metalloproteins	Metal	Nitrate reductase — an enzyme that catalyzes the reduction of nitrate to nitrite in plants

Table 3.9. Classification of proteins by function. Proteins also play a crucial role in membranes, where they function as enzymes, receptors, and transport proteins

Protein class	Examples	Localization/function
Structural proteins	Collagen Keratin Elastin Viral capsid proteins	Component of connective tissue, bones, tendons, Cartilage Skin, feathers, nails, hair, horns Elastic connective tissue (ligaments) Protective coat of viral nucleic acid
Enzymes	Trypsin Ribulose bisphosphate carboxylase Glutamine Synthetase	Catalyzes Protein Hydrolysis Catalyzes the carboxylation (addition of CO₂) of ribulose bisphosphate during Photosynthesis Catalyzes The formation of The amino acid glutamine from glutamic acid and ammonia
Hormones	Insulin Glucagon ACTH	Regulates glucose METABOLISM Regulates glucose metabolism Stimulates the growth and activity of the adrenal cortex
Respiratory pigments	Hemoglobin Myoglobin	Transports O₂ in vertebrate blood Serves for O₂ storage in muscles
Transport proteins	Serum albumin	Serves for The transport of Fatty acids and lipids in the blood
Protective proteins	Antibodies Fibrinogen Thrombin	Form complexes with foreign proteins Precursor of fibrin in blood clotting Involved in the blood clotting process
Contractile proteins	Myosin Actin	Mobile filaments of sarcomere myofibrils Stationary filaments of sarcomere myofibrils
Storage proteins	Ovalbumin Casein	Egg protein Milk protein
Toxins	Snake venom	Enzymes
	Diphtheria toxin	Toxin produced by the diphtheria bacterium

Last update: 06/08/2026

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