Globular protein examples comprise a group of proteins that have their polypeptide chains arranged in a spherical form.
- Carbonic anhydrase
- Glyceraldehyde-3-phosphatase dehydrogenase
- Concanavalin A
- Cytochrome C
Myoglobin is the first globular protein to have its tertiary structure determined. This small, oxygen carrying protein is made of single polypeptide chain of 153 amino acids and a non-polypeptide prosthetic heme group. About 75% of the amino acid residue in myoglobin is α helical. Myoglobin lacks β sheet structures.
Globular protein human carbonic anhydrase is an enzyme that assist in O2 -CO2 transport. It has a single polypeptide chain of 259 amino acids with a Zn2+ ion in the center coordinated by Histidine amino acid residue in position 94,96,119. This is the only native protein in which polypeptide chain form a knot by the C-terminus going through surrounding polypeptide loop.
This globular protein is an important enzyme needed in glycolytic pathway and contains total of 335 amino acid residues. It is made of four identical subunits of 37 kDa, each containing a thiol(SH) group. It has two highly conserved domains, I.e., NAD+ binding domain and glyceraldehyde-3-phosphate domain. It contains a unique CREM nuclear export signal at the catalytic site. GAPDH also helps in apoptosis.
This globular protein is a homotetramer with each subunit containing 235 amino acids and weighs 26.5 kDa. They are mostly consisting of antiparallel β sheets and non-polypeptide prosthetic Mn2+ and Ca2+.
This small globular protein, which is made of 104 amino acid residues and heme group, is an integral part of electron transport chain. The heme group is attached covalently to the polypeptide chain. 40% of the polypeptide of this protein form α helical structure and the rest contains β turns. Except ETC this protein also helps in cellular apoptosis.
This small proteolytic enzyme found in human tears, saliva, mucus is a globular protein made of 129 amino acid residues. It contains four disulfide bonds, which provides structural stability, in-between the pairs of cysteine residues. It has five regions of α helices, about 40% of the polypeptide chain and the rest shows β sheet conformations. Lysozyme is well known for the ability to lyse or degrade microbial cell walls.
Most globular proteins are water soluble.
While forming tertiary structure polypeptide chains fold in such a manner where the hydrophilic polar amino acids are found in the outer region whereas the core is formed by non-polar hydrophobic amino acids.
Primary and secondary structure of protein–
Primary structure of protein is a polymer made of amino acids linked by peptide bonds and are called polypeptide chains. Polypeptide chains fold into different stable structures; like α helix, β pleated sheets and u-shaped β turn, depending on amino acid sequences. These structures are known as the secondary structure of protein.
Tertiary structure of globular protein-
Most proteins found inside cells; like transport and receptor proteins, enzymes and immunoglobins are globular proteins. Polypeptide chains fold back on each other forming a compact structure that varies from protein to protein. These structural diversity helps protein maintain their unique structure to perform assigned biological functions.
3-dimensional protein structure is identified using x-ray crystallography and NMR spectroscopy.
Globular proteins are not only important for structural stability of tertiary protein molecules, also is imperative for specific biological function that is determined through conserved regions of tertiary structure of protein.