Explore what makes globular proteins different from other groups of proteins. Learn about the definition and structure of globular proteins as well as some examples and their functions.
What Are Globular Proteins?
Did you enjoy your scrambled eggs this morning? Or, perhaps you prefer your eggs fried sunny-side up or folded with ham and cheese into a delicious omelet. Eggs, specifically egg whites, contain a type of protein called albumin, or ovalbumin. Ovalbumin is an example of a globular protein, one that is water-soluble and shaped roughly like a globe or a sphere when coiled up into its functional form.
Let’s explore more about globular proteins and their structure.
Based on solubility, function, and three-dimensional shape, proteins may be classified as fibrous, membrane, or globular. Globular proteins, like ovalbumin, are always soluble in water and often have a spherical or roundish shape.
As in all proteins, the primary structure of globular proteins consists of a polypeptide, or chain of amino acids joined via peptide bonds. Hydrogen bonds between carboxyl and amine groups of the amino acids contribute to the secondary structure, which in globular proteins may include alpha-helices, beta-sheets, or both. Globular proteins are folded such that their tertiary structure consists of the polar, or hydrophilic, amino acids arranged on the outside and the nonpolar, or hydrophobic, amino acids on the inside of the three-dimensional shape. This arrangement is responsible for the solubility of globular proteins in water.
Some globular proteins, such as hemoglobin, also exhibit quaternary structure, in which multiple polypeptide chains are clustered together to form a functional protein. The shape of globular proteins is critical to their function. Those acting as enzymes, for example, must be folded such that their active sites are in the correct orientation and conformation to interact with their substrates.Changes in environmental conditions, such as pH or temperature, may affect the bonds that contribute to the shape of globular proteins. These changes can lead to denaturation in which the shape of the protein is so altered that it is no longer functional.
In the case of your eggs at breakfast, the heat used in cooking the eggs denatures the ovalbumin and other proteins in the egg whites, causing them to unfold and clump together into a yummy conglomeration of goodness.
Globular proteins play many biological roles, including acting as enzymes, hormones, immunoglobulins, and transport molecules. Hemoglobin is a globular protein found in red blood cells. It is made of four polypeptide chains, each containing a heme group that binds and transports oxygen through the blood stream.
Skeletal muscle contains a protein called myoglobin, a single protein chain also containing a heme group that binds oxygen for release when muscles are working hard. Myoglobin is what makes ‘red meat’ look red.Immunoglobulins like IgG and IgA, are globular proteins that act as antibodies to help your immune system battle infections from things like viruses and bacteria. Another example is the enzyme amylase, which helps us break down starch into sugar during digestion.Insulin is another globular protein that acts as a hormone.
It allows cells to take in and utilize glucose. The hormone human chorionic gonadotropin, or hCG, is an example of a globular protein that doesn’t quite look globular, but it is still folded with the same arrangement of hydrophilic and hydrophobic amino acids so that it is water soluble. Ovalbumin makes up over 50% of the protein in egg whites, and while its function is currently unknown, we sure don’t mind eating it!
The group known as globular proteins is one of the three main protein groups based on shape and structure. All globular proteins are water-soluble, with their tertiary level of structure often resembling a globe or sphere. This ‘globular’ folded shape is crucial to the function of these proteins and their ability to serve a variety of biological roles. Examples of globular proteins include hemoglobin, myoglobin, immunoglobulins, amylase, insulin, and hCG.