Difference Between Integral and Peripheral Proteins

Main Difference – Integral vs Peripheral Proteins

Plasma membrane is the biological membrane which separates the contents of the cell from the outside environment. It is made up of a phospholipid bilayer. Plasma membrane serves as a selectively permeable barrier which only allows certain molecules to pass through the membrane. Different types of proteins are embedded in the plasma membrane as well. Integral and peripheral proteins are two types of such membrane proteins. The main difference between integral and peripheral proteins is that integral proteins are embedded in the whole bilayer whereas peripheral proteins are located on the inner or outer surface of the phospholipid bilayer.

Key Areas Covered

1. What are Integral Proteins
      – Definition, Structure, Function
2. What are Peripheral Proteins
      – Definition, Structure, Function
3. What are the Similarities Between Integral and Peripheral Proteins
      – Outline of Common Features
4. What is the Difference Between Integral and Peripheral Proteins
– Comparison of Key Differences

Key Terms: Integral Proteins, Hydrophobic Core, Membrane Proteins, Peripheral Proteins, Phospholipid Bilayer, Plasma Membrane, Transmembrane Protein

What are Integral Proteins

Integral proteins are proteins that are permanently attached to the plasma membrane. They penetrate through the phospholipid bilayer. Therefore, these proteins are attached to the lipid bilayer through hydrophobic, electrostatic or non-covalent interactions. On that account, detergents should be used to remove integral proteins from the lipid bilayer. This will destroy the hydrophobic interactions of the whole lipid bilayer. Transmembrane proteins, which completely penetrate the lipid bilayer, are integral proteins. All transmembrane proteins are integral proteins, but all integral proteins are not transmembrane proteins. This means some integral proteins can partially penetrate the lipid bilayer.

Figure 1: Transmembrane Protein

Since most integral proteins are transmembrane proteins, they serve as membrane transport proteins such as carrier proteins and channel proteins. Other integral proteins serve as receptors and enzymes. Na+ – K+ ATPase, ion channels and gates, glucose permease, hormone receptors, histocompatibility antigens, certain enzymes of the electron transport chains, and gap junction proteins are types of integral proteins.

What are Peripheral Proteins

Peripheral proteins are proteins that are temporarily attached to the plasma membrane. This means they do not penetrate the hydrophobic core of the phospholipid bilayer. Most peripheral proteins are associated with the proteins. Thereby, the removal of peripheral proteins is easier than integral proteins. The removal does not interrupt the hydrophobic interactions of the lipid bilayer as well. The structure of both integral and peripheral proteins are shown in figure 2.

Figure 2: Integral and Peripheral Proteins

Most peripheral proteins serve as receptors, antigens, and enzymes. Cytochrome c in the electron transport chain is an example of a peripheral protein.

Similarities Between Integral and Peripheral Proteins

• Both integral and peripheral proteins are two types of membrane proteins, which are attached to the plasma membrane.
• Both integral and peripheral proteins contain hydrophilic components.
• Both integral and peripheral proteins perform important functions in the cell.

Difference Between Integral and Peripheral Proteins

Definition

Integral Proteins: Integral proteins are proteins that are permanently attached to the plasma membrane.

Peripheral Proteins: Peripheral proteins are proteins that are temporarily attached to the plasma membrane.

Alternative Names

Integral Proteins: Integral proteins are called intrinsic proteins.

Peripheral Proteins: Peripheral proteins are called extrinsic proteins.

Location

Integral Proteins: Integral proteins are embedded in the whole membrane.

Peripheral Proteins: Peripheral proteins are located on the inner or outer surface of the phospholipid bilayer.

Interaction with the Hydrophobic Core of the Lipid Bilayer

Integral Proteins: Integral proteins highly interact with the hydrophobic core of the lipid bilayer.

Peripheral Proteins: Peripheral proteins interact less with the hydrophobic core of the lipid bilayer.

Types of Interactions with Lipid Bilayer

Integral Proteins: Integral proteins bind to the lipid bilayer by hydrophobic, electrostatic or non-covalent interactions.

Peripheral Proteins: Peripheral proteins on the inner surface of the lipid bilayer are held by the cytoskeleton.

Constituent of the Membrane Protein

Integral Proteins: Integral proteins constitute 70% of the total membrane proteins.

Peripheral Proteins: Peripheral proteins constitute 30% of the total membrane proteins.

Hydrophilic/Hydrophobic

Integral Proteins: Integral proteins contain both hydrophilic and hydrophobic parts.

Peripheral Proteins: peripheral proteins contain hydrophilic parts.

Function

Integral Proteins: Integral proteins serve as carrier proteins, channel proteins, and enzymes.

Peripheral Proteins: Peripheral proteins serve as receptors and surface antigens.

Protein Removal

Integral Proteins: Detergents should be used to remove integral proteins from the plasma membrane.

Peripheral Proteins: Dilute salt solutions can be used to remove peripheral proteins from the plasma membrane.

Examples

Integral Proteins: Glycophorin, rhodopsin, and NADH dehydrogenase are examples of integral proteins.

Peripheral Proteins: Mitochondrial cytochrome c and erythrocyte spectrin are examples of peripheral proteins.

Conclusion

Integral and peripheral proteins are two types of membrane proteins in the phospholipid bilayer. Integral proteins penetrate the hydrophobic core of the lipid bilayer while peripheral proteins are attached to the intracellular or extracellular surface of the lipid bilayer. Transmembrane proteins are a type of integral proteins. The main difference between integral and peripheral proteins is the penetrance of the hydrophobic core of the lipid bilayer.

Reference:

1. Lodish, Harvey. “Membrane Proteins.” Molecular Cell Biology. 4th edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
2. “Integral membrane proteins.” Integral membrane proteins, Available here.
3. “Peripheral membrane protein.” Peripheral membrane protein, Available here.

Image Courtesy:

1. “Transmembrane receptor” By Mouagip (talk) (CC BY-SA 3.0) via Commons Wikimedia
2. “Membrane protein” By Meng-jou wu at English Wikibooks – Transferred from en.wikibooks to Commons by Adrignola (Public Domain) via Commons Wikimedia