What are the two common types of proteins?

October 30, 2025 •

3 min read

Over 30% of your body's total protein content is composed of the fibrous protein, collagen, but all proteins can be broadly classified into two main categories: what are the two common types of proteins? These are fibrous and globular proteins, which differ significantly in shape, function, and solubility.

Quick Summary

Proteins are structurally classified into two main types: fibrous and globular. The key distinctions lie in their shape, function, stability, and water solubility. Fibrous proteins, like keratin, provide structural support, while globular proteins, such as hemoglobin, have metabolic roles.

Key Points

Fibrous proteins: These are long, narrow, and tough protein strands that provide structural support and strength, primarily in tissues like hair, skin, and bones.
Globular proteins: These are compact, rounded, and typically water-soluble proteins that perform dynamic metabolic functions, such as enzymatic catalysis and transport.
Shape dictates function: The elongated shape of fibrous proteins makes them ideal for structural roles, while the spherical shape of globular proteins is crucial for their mobility and specific binding sites.
Solubility differences: A key distinction is that fibrous proteins are generally insoluble in water, whereas globular proteins are soluble, allowing them to move within cells and body fluids.
Key examples: Collagen and keratin are prominent fibrous proteins, while hemoglobin, insulin, and enzymes are common examples of globular proteins.
Environmental stability: Fibrous proteins are relatively stable against changes in temperature and pH, whereas the functional shape of globular proteins is more sensitive to these environmental factors.

Protein Classification

At a fundamental level, proteins can be categorized based on their three-dimensional shape and function. The two primary classifications are fibrous and globular proteins, each with unique characteristics that dictate their role within an organism. These proteins are made up of amino acid chains, and the way these chains fold and interact determines the final structure and properties of the protein. The distinction between fibrous and globular proteins is crucial for understanding a vast array of biological processes, from structural integrity to enzymatic reactions.

Fibrous Proteins: The Structural Backbone

Fibrous proteins are long, thread-like molecules that play a crucial role in providing structural support and strength to cells and tissues. Their elongated shape results from polypeptide chains arranged in parallel to one another, often forming strong, insoluble fibers or sheets.

Characteristics of Fibrous Proteins

Shape: Long and narrow strands.
Function: Primarily structural, providing strength and support.
Solubility: Generally insoluble in water.
Sequence: Repetitive amino acid sequence.
Stability: Less sensitive to changes in temperature and pH.

Examples of Fibrous Proteins

Collagen: The most abundant protein in mammals, forming the main component of connective tissues like tendons, ligaments, and skin. It possesses a characteristic triple helix structure.
Keratin: Found in hair, nails, and the outer layer of skin, α-keratin is composed of coiled-coil α-helices, while β-keratin is made of β-sheets found in feathers and scales.
Elastin: A protein found in connective tissue, particularly in elastic tissues like the walls of arteries, allowing them to stretch and recoil.

Globular Proteins: The Functional Powerhouses

In contrast to fibrous proteins, globular proteins are compact, spherical molecules. Their polypeptide chains are folded into complex tertiary and quaternary structures, giving them a more rounded shape. This compact structure allows them to be water-soluble, making them mobile within a cell and enabling them to perform dynamic functions.

Characteristics of Globular Proteins

Shape: Compact, rounded, or roughly spherical.
Function: Primarily functional, acting as enzymes, hormones, and transporters.
Solubility: Typically soluble in water.
Sequence: Irregular amino acid sequence.
Stability: More sensitive to changes in temperature and pH.

Examples of Globular Proteins

Hemoglobin: This protein transports oxygen in red blood cells. Its compact shape allows it to fit efficiently within the cells and bind reversibly to oxygen.
Enzymes: Nearly all enzymes, such as amylase and trypsin, are globular proteins. Their precise, specific three-dimensional shape creates active sites that catalyze biochemical reactions.
Insulin: A hormone that regulates blood glucose levels, insulin's soluble, globular form allows it to travel freely in the bloodstream to reach its target cells.

Comparison of Fibrous and Globular Proteins


Feature	Fibrous Proteins	Globular Proteins
Shape	Long and narrow strands	Compact and spherical
Function	Structural (e.g., support, strength)	Functional (e.g., catalysis, transport)
Solubility	Generally insoluble in water	Typically soluble in water
Amino Acid Sequence	Highly repetitive	Irregular and complex
Stability	Mechanically strong, less sensitive to environmental changes	More sensitive to changes in temperature and pH
Examples	Keratin, Collagen, Elastin	Hemoglobin, Enzymes, Insulin

The Interplay of Structure and Function

The distinct structures of fibrous and globular proteins directly relate to their biological roles. The high tensile strength and insolubility of fibrous proteins, for example, make them ideal for creating strong connective tissues and protective coverings like hair and skin. Their elongated shape, formed by parallel polypeptide chains, provides the necessary rigidity and durability for structural integrity.

On the other hand, the compact, water-soluble nature of globular proteins is essential for their metabolic functions. Being mobile within the cell and bloodstream, they can act as efficient transporters, chemical messengers, and catalysts, all of which require a specific, folded shape to interact with other molecules. The specific folding of globular proteins is what allows for the precise binding in enzymes and the reversible attachment of molecules like oxygen in hemoglobin. Changes to a globular protein's temperature or pH can disrupt this fragile structure, causing it to lose its function, a process known as denaturation.

Conclusion

Understanding the classification of proteins into fibrous and globular types reveals how their fundamental molecular structure dictates their role in living organisms. Fibrous proteins are the architects, providing the robust framework and strength necessary for structural components like hair, skin, and connective tissues. Globular proteins are the busy workers, performing the dynamic, metabolic tasks vital for survival, from transporting oxygen to catalyzing chemical reactions. The incredible diversity of proteins, all built from the same 20 amino acids, is a testament to the elegant relationship between form and function in biochemistry. For further reading on protein structure, a good resource is Chemistry LibreTexts.

Frequently Asked Questions

Common examples of fibrous proteins include collagen, which is a major component of skin and connective tissue; keratin, found in hair and nails; and elastin, which gives elasticity to tissues and organs.

Well-known examples of globular proteins are hemoglobin, which transports oxygen in blood; enzymes like amylase and pepsin, which catalyze reactions; and hormones such as insulin, which regulate metabolic processes.

The main functional difference is that fibrous proteins are primarily structural, providing strength and support, while globular proteins are functional, performing metabolic roles like transport, regulation, and catalysis.

Globular proteins are typically soluble in water. This is due to their folded, spherical shape, which positions hydrophilic (water-loving) amino acid side chains on their exterior.

A protein's shape is critical to its function. The linear, elongated shape of fibrous proteins provides tensile strength for structural roles. The specific compact, spherical shape of globular proteins creates precise active sites for binding and catalyzing reactions.

Enzymes are globular proteins because their folded, three-dimensional shape creates a specific active site. This site is crucial for binding to substrates and catalyzing reactions with high specificity and efficiency.

If a protein loses its specific three-dimensional shape, a process called denaturation, it will also lose its function. This can happen due to changes in temperature or pH, which are particularly damaging to the fragile structure of globular proteins.