What are Globular Proteins?

The globular proteins refer to the shape of protein specifically spherical in nature apart from spherical form fibrous, disordered and membrane-bound proteins exist. These globular proteins are miscible in water and form a colloidal solution rather than other types which might not exhibit solubility. Many classes of the fold are found in globular proteins, which render them a sphere shape. Globular fold containing proteins usually are referred to by the term globin.

Structure

The name globular protein is outdated and now obsolete. It has been used to name hundreds and thousands of proteins and is mainly a more graceful motif vocabulary used commonly. The techniques like ultracentrifugation or Dynamic Light Scattering (DLS) are used to determine the nature of globular proteins without the help of updated technologies. The tertiary structure of the protein brings on a spherical shape to the protein. The solubility of the proteins can be illustrated by a nonpolar and polar amino acid forming the primary structure of the protein. The nonpolar or hydrophobic amino acid is buried inside the protein structure, and the polar or hydrophilic amino acids are arranged to face outwards. This arrangement of polar amino acids in a protein facilitates the dipole-dipole interaction with the solvent environment.

There are other forces such as hydrogen bonding, van der Waals interaction, covalent and noncovalent bonding that help to maintain the tertiary structure of the globular protein.  The disulfide bonds from chelated metal ions of certain unstructured segments are notable despite the backbone layers present. This context aims to emphasize the structural details of globular proteins by briefing their characteristics.

Features

The globular proteins are less stable due to the release of very small free energy while folding towards the native condition. The low free energy is because the entropic cost required for protein folding is high. The native globular structure conformations are restricted, whereas the primary polypeptide sequence can take up many forms. This results in the reduction of randomness and stabilization of structure by noncovalent interactions like hydrophobic interactions.

Apart from these interactions, several non-covalent van der Waals interactions like hydrogen bond facilitate folding, yet analyzed in the protein folding problem. Many techniques are employed for studying the protein folding mechanism. Even the denatured protein state can be folded into a properly folded state.

Two main mechanisms of protein folding have been postulated, namely, the diffusion collision model and nucleation condensation model. The folding of certain proteins such as PTP‐BL PDZ2 uses both the mechanisms (diffusion collision model and nucleation condensation model) for proper folding. These findings suggest that protein folding can readily be affected by the transition states of the proteins. Globular protein folding plays a major role in treating certain diseases, where the anti-cancer ligand interacts with the folded protein but fails to bind to the unfolded protein. 

As per the second law of thermodynamics, the free energy difference between folded and unfolded states is accounted for by their enthalpy and entropy changes. The globular protein liberates less free energy during folding to its native state resulting in a less stable state, which provides a rapid turnover rate and control in the degradation of protein and synthesis.

Importance

Unlike other proteins such as fibrous proteins which play an important role in structural function, globular protein takes up different roles and is highly flexible and variable in nature. As enzymes, globular proteins catalyze various organic reactions in mild conditions with high specificity. They can also act as messenger proteins by transmitting messages for regulation of the biological process. Hormones such as insulin can act as messenger proteins which signal the target cells to uptake glucose molecules from the blood. They also act as transporters which help in the movement of molecules through the plasma membrane.

Globular protein majorly plays a regulatory role as well as some structural functions. The structural proteins like actin, tubulins are also globular and are soluble as monomers but after polymerization, they become stiff fibers. The three-dimensional structure of globular protein ranges from sphere to cigar shape. The structural level of globular protein is divided into three to four levels:

• The sequence of the amino acids which results in the formation of polypeptides is referred to as the primary structure. These primary structures when folded properly and repetitively, form secondary structures.
• The secondary structure comprises three forms, namely helix, sheets (parallel and antiparallel), and turns. Turns are classified as secondary structures as they are in an orderly manner but nonrepetitive. The secondary structure represents the two-dimensional structure of the protein.
• The tertiary structure is the three-dimensional protein model comprising all secondary structure elements with certain folds formed by turns and loops.
• The non-covalent attractions govern the folded conformation of the globular protein. These attractions are contributed mainly by the side chain of the amino acid or between the side chains and bound ligands.
• The backbone hydrogen bonds and side chains hydrogen bonds are important in maintaining secondary and tertiary structures, respectively.

Members of Globular Proteins

Many important proteins are globular in structure and include immunoglobulins (IgA, IgM, IgD, IgE, and IgG) and hemoglobin. Immunoglobulins are an important component of the adaptive immune system and help the body fight against pathogens and allergens.

Hemoglobin is an oxygen-carrying protein. It carries oxygen to different cells and tissues through the blood which uses it to generate ATP (adenosine triphosphate). The ATP act as an energy currency important for the survival of the cells and tissues. The globular structure of hemoglobin plays a major role in the transport of oxygen by the blood.

Myoglobin is the oxygen storing protein present in the muscles. Most of the enzymes and signal-transducing elements possess globular structures, which are important for their catalytic activity. Albumin is an important protein synthesized by the liver and is the only water-soluble globular protein. It is insoluble in oil.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biochemistry
• Master of Science in Biochemistry
• Bachelor of Science in Life Sciences
• Master of Science in Life Sciences

Related Concepts

• Secondary structures
• Tertiary structure and quaternary structure
• Fibrous proteins
• Hemoglobin and myoglobin