The key difference between G protein-linked receptors and enzyme-linked receptors is that G protein-linked receptors bind with an extracellular ligand and activate a membrane protein called a G-protein while enzyme-linked receptors bind with an extracellular ligand and cause enzymatic activity on the intracellular side.
In multicellular organisms, cells communicate with each other via chemical signals. Cells send messages as well as receive messages. Through these messages, all the activities occurring within the organism are coordinated. Paracrine, endocrine, autocrine, and direct signalling are the four major types of cell signalling mechanisms. Cells receive signals through receptors. These receptors can be intracellular receptors or cell surface receptors. Intracellular receptors are present in the cytoplasm, while cell surface receptors are present on the exterior side of the cell membrane. There are three main types of cell surface receptors as ion channel linked receptors, G protein-linked receptors and enzyme-linked receptors.
CONTENTS
1. Overview and Key Difference
2. What are G Protein Linked Receptors
3. What are Enzyme-Linked Receptors
4. Similarities Between G Protein Linked Receptors and Enzyme-Linked Receptors
5. Side by Side Comparison – G Protein Linked Receptors vs Enzyme-Linked Receptors in Tabular Form
6. Summary
What are G Protein Linked Receptors?
G protein-linked receptors are a type of transmembrane proteins. As their name suggests, these receptors work with the G proteins that associate with GTP. GTP is a molecule like ATP that provides energy for G proteins to work. When a ligand binds with the G protein-linked receptor, it undergoes a conformational change in a way that it can interact with the G protein.
Figure 01: G Protein-linked Receptors
The inactive form of the G protein transforms into the active form and divides into two pieces (alpha and beta subunits) by converting GTP into GDP and utilizing the released energy. These subunits then separate from G protein-coupled receptor and interact with other proteins to trigger cell responses. Structurally, G protein-coupled receptors have seven transmembrane domains that span across the membrane.
What are Enzyme-Linked Receptors?
Enzyme-linked receptors are another type of cell surface receptors or transmembrane receptors. When an extracellular ligand binds with an enzyme-linked receptor, this binding causes enzymatic activity inside the cell. An enzyme activates and sets off a chain of events within the cell that eventually leads to a response. Therefore, these receptors have an intracellular domain that associates with an enzyme. In some cases, this intracellular domain itself works as an enzyme, or it interacts directly with an enzyme. Structurally, enzyme-linked receptors have large extracellular and intracellular domains and a membrane-spanning single alpha-helical region.
Figure 02: Enzyme-linked Receptors
Receptor tyrosine kinase is an enzyme-linked receptor. It is a type of receptor protein involved in most of the cell signalling pathways. As its name implies, receptor tyrosine kinases are kinase enzymes. Kinase is an enzyme that catalyzes the transfer of phosphate groups to a substrate. These receptors contain tyrosine kinases that transfer a phosphate group from ATP to tyrosine.
Receptor tyrosine kinase has two similar monomers. Once a signalling molecule binds with the binding site of the receptor, two monomers come together and form a dimer. Then, kinases phosphorylate ATP and add phosphate groups to each of the six tyrosines. Hence, dimer becomes phosphorylated, which is a fully activated tyrosine kinase. Activated tyrosine kinase sends signals to other molecules of the cell and mediates signal transmission. The most important characteristic of receptor tyrosine kinase is that it can activate multiple signalling pathways, and when it activates, it can create multiple cell responses at once.
What are the Similarities Between G Protein Linked Receptors and Enzyme-Linked Receptors?
- G protein-linked receptors and enzyme-linked receptors are two categories of cell surface receptors.
- They are transmembrane proteins.
- They are specific to individual cell types.
- Initially, they remain inactive and then become active upon binding a ligand to the receptor.
What is the Difference Between G Protein Linked Receptors and Enzyme-Linked Receptors?
G protein-linked receptors are the cell surface receptors that activate G proteins upon binding with an extracellular ligand. In contrast, enzyme-linked receptors are the cell surface receptors that activate with an enzyme and sets off a chain of events within the cell. So, this is the key difference between G protein-linked receptors and enzyme-linked receptors. Moreover, G protein-linked receptors have seven transmembrane domains spanning across the membrane while enzyme-linked receptors have a membrane-spanning a single alpha-helical region.
The below infographic provides more comparisons relating the difference between G protein-linked receptors and enzyme-linked receptors.
Summary – G Protein Linked Receptors vs Enzyme-Linked Receptors
G protein-linked receptors and enzyme-linked receptors are two types of transmembrane receptors. G protein-linked receptors bind with an extracellular ligand and activate a membrane protein called G protein. G protein activation triggers cell responses. On the other hand, enzyme-linked receptors bind with extracellular ligands and activate enzymes which trigger a chain of events within the cell that eventually leads to a response. Therefore, the intracellular domains of these receptors associate with enzymes. So, this is the summary of the difference between G protein-linked receptors and enzyme-linked receptors.
Reference:
1. “Signaling Molecules and Cellular Receptors.” Lumen, Available here.
2. Purves, Dale. “Receptor Types.” Neuroscience. 2nd Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
Image Courtesy:
1. “G protein” By Tpirojsi – Own work (Public Domain) via Commons Wikimedia
2. “Figure 09 01 07” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia