Structure and Functions of Cytoplasmic Actin
Cytoplasmic actins refers to the conserved proteins. Actin can be found in all eukaryotic cells, and are involved in variety of cell motility. Protein structure is formed by combination of variety of amino acids by losing the molecule of water. Proteins get involved into variety of biological functions, and proteins have different types of structures depending upon the requirements. This paper intends to discuss different types of protein structures, their functions and other related aspects.
The structure of proteins can be divided into three categories i.e. primary, secondary and tertiary. The structures of proteins are polypeptides-sequences that get formed by arrangements of L-α-amino acids. Interaction with different chemical elements resulted into formation of different type’s forces and bonding such as hydrogen bonding and van der waals forces. The functions of the proteins can easily understand after identifying the different types of structures of the protein. Protein structures differ in size, shape and arrangements. Physically proteins are called nanoparticles that ranges between 1 to 100 nm. A large protein is formed when multiple protein subunits get joint. Huge number of actin molecules formed microfilament by assembling in an order (Remedios & Chhabra, 2006).
Primary structure of protein indicates towards a unique chain of amino acids in the series of polypeptides. When two or more amino acids are placed in a situation wherein they can react collectively, a peptide formation takes place. In a polypeptide, amino acids are linked with peptide bonds, starting with N-terminal and ending with C-terminal. Peptide bonds are unable to rotate in a free manner because of double bond character of peptide bonds. A polypeptide of an average length is made of three hundred amino acids while long polypeptides are made of thousands of amino acids. Primary structure of a protein is important because of its capability to encode patterns that play very crucial role in biological functions.
The specific sequence formed by amino acid in a polypeptide causes polypeptides to form various other compact structures. Amino acids tend to rotate around peptide bonds inside a protein. Due to this reason, proteins are flexible and they can fold easily and can take various shapes. However such shapes and folding can be irregular. The folds and coils, formed in a polypeptide bond are known as secondary structure of protein. These individual bonds are usually weak but since they are in a repeated structure, they make a strong bond. A-helix, B-sheets and B-turns are three most important secondary structures.
Tertiary structure of the protein get formed when primary and secondary proteins clubbed together, or secondary structure of protein get fold from different regions. Tertiary structure is a three dimensional 3D arrangement which includes b sheet, loops, helices, fold, and polypeptide chain. In tertiary structure, side chains either interacts with each other, or interact with polypeptide backbone. The folding pattern of protein varies from protein to protein and very complex in nature. A polypeptide chain consists of both hydrophilic residues and hydrophobic residues.
Secondary structure of protein is more stable due to H-bonding, but tertiary structure is weak due to weak forces. Hydrophobic interaction refers to one of the major force. Protein generally becomes stable when hydrophobic parts get suppressed, or on the surface, or exposed to the water. Therefore, trp generally found surrounded with some parts of protein but not water, and asp which is a charged residues found on the external surface. In physiologic situations, side chain of neutral’s hydrophobic, non-polar amino acids inclined to get buried on inner side of protein molecule in order to protect protein from water or aqueous medium (Carlier, 2010).
Disulfide bridges get formed when sulfhydryl groups oxidize on cysteine, and this stage plays important role in stabilization of tertiary structure of the protein. Formation of disulfide bridges allows all protein chain arts to bind together covalently. Hydrogen bonds can also get formed between among side-chain groups. Salt bridges also facilitate stabilization of the tertiary protein structure when different sites of amino acid i.e. positive and negative get interacted with each other.
Actin is major cytoskeletal protein found in majority of the cells. Actin polymerizes in order to develop actin filaments. Filaments are thin and flexible with around 7nm diameter. Actin filaments forms higher-order structures such as bundles and 3D networks by arranging in different sequences. Different structures of filament carries semisolid gel properties. The assembly and de-assembly of filaments occurs by crosslinking into networks and filament bundles. Filaments linking with different cell structures is regulated by actin binding proteins; such proteins are important and essential part of actin cytoskeleton. The presence of actin filaments can be observed under the plasma membrane. The network formed by actin filaments provides mechanical support, allow cell movement, and allow cells to relocate, provide shape to the cell, and facilitate their division (Alberts et al., 2002).
Actin was initially extracted from the muscle cell, and it formed around 20 percent of the total cell protein. Actin protein is found in all types of eukaryotic cells. In mammals around six different types of actin protein have found, out six four were related to muscles and two were to other cells. The three-dimensional structures of single actin and also actin filament (bundle and networks) were identified in year 1990 by Holmes and his colleagues (GM., 2000).
Actin monomer also refers as globular actin has tightly bounded sites that facilitate head-to-tail interactions of the monomer with other actin monomers. Therefore, polymerization of each actin monomer occurred in order to form filaments. Every single actin monomer is formed by 375 amino acids (globular proteins). While forming filaments actin monomers are required to rotate by 166 degree in filaments; this has appearance like double stranded helix. This appear because actin monomers oriented in one direction, and actin filaments carries unique polarity towards their ends (positive and negative) which makes both ends distinguish (GM., 2000).
The assembly and disassembly of actin filaments occurred due to polarity at the ends of the filaments. The polarity also provides support in creating unique myosin movement in single direction with regard to actin. In assembly first step refers to formation of dimers & trimmers, which further grow to form filaments by adding more actin monomers. The disassembly of the actin filaments occurred when filament get divided into two or multiple parts. Disassembly occurs due to reaction actin with other substances or due to internal reaction such as interaction within the filament (Schliwa, 1981).
Proteins are very complex structured as the formation and chemistry of protein has developed over millions of years. Protein is shaped as per specific sequence of amino acids. There are many amino acids in proteins and every one of them has different compound properties. Protein molecules are made of a unique chain of amino acids. This chain is connected to each other with the help of peptide bonds. This is the reason why proteins are also known as polypeptides.
Protein is located in membrane of cell and from there it keeps operating its several functions. Protein has several functions and depends on its property in order to look after various functions. Its location and compound property helps it in fulfilling all its functions in a smoother manner. Protein is the main agent of binding molecules. Protein binds other molecules and this is one of other important functions of protein. Protein repairs and maintains various components. It takes care of various parts of the human body. It gives energy and releases various enzymes that are necessary for proper functioning of human body.
Having observed an overview and discussion of the above mentioned subject, the paper concludes that protein is very significant substance. It has evolved as a result of various reactions over millions of years. Protein has a complex structure and it is made of amino acids. Protein has several important functions and it is equally important for humans and animals. The structure and function of protein are very complex and the reason of such complexity is its evolution and continuous development.
References
Alberts, B, Johnson, A, and Lewis, J, , 2002. Molecular Biology of the Cell. 4th ed. New York: Garland Science.
Carlier, M.-F., 2010. Actin-based Motility: Cellular, Molecular and Physical Aspects. New York: Springer Science & Business Media.
GM., C., 2000. The Cell: A Molecular Approach. 2nd ed. Sunderland, MA: Sinauer Associates.
Remedios, C.D., & Chhabra, D., 2006. Actin-Binding Proteins and Disease. 8 ed. NewYork: Springer Science & Business Media.
Schliwa, M., 1981. Proteins associated with cytoplasmic actin. Cell, 25(3), pp. 587-590.
