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Proteins are very important complex molecules that play significant biological functions in the cells that are most important to life (growth, repair, regeneration, etc). Proteins are polymers made up of amino acids (building blocks) which differ to other molecules due to their nitrogen content, and are linked by peptide bonds. The structure of protein is dependent on the amino acid sequence (primary structure) which determines the molecular conformation (secondary and tertiary structures). Amino acids are very soluble, due to presence of oxygen and nitrogen in them is very electronegative (amino acids are readily carried around in aqueous state).Proteins can also occur as quaternary structures .Primary structure gives the linear sequence of amino acids in a polypeptide chain while the secondary structure reveals the arrangement of the chain in space (polypeptide chain coiled into a spiral or helix to have a three dimensional structure, alpha helix). Tertiary (disulphide bond) (helical polypeptide molecule is folded on itself as spherical or rod like) and quaternary structured proteins can be dived into two groups on basis of conformation; fibrous or scleroproteins, and globular or folded proteins.
In this essay I will begin by talking about the structure of proteins and will go on to discuss about their functions. Three main protein examples will be used; casein, ovalbumin and collagen protein.
Structure of proteins
The primary structure of a protein defines a key location called the active site, the region that is associated with the primary activity of a protein, even though it is often made up of only a small number of residues. Therefore, the amino acid unique linear sequence in polypeptide chain defines the protein primary structure. Primary structure is composed of various amino acids held together by peptide bonds.
Secondary structure makes 60% of the protein structure consists of alpha helix or beta strand (sheets) conformation. It is most common spiral structure of protein form right handed helix (right handed arrangement). Each turn of a alpha helix has 3.6 amino acids. This coiled structure is locally stabilized by hydrogen bonding which occurs between C=O group of one amino acid and N-H group of fourth amino acid in the chain, disulphide bond, attraction between positive and negative charges, and hydrophobic and hydrophilic radical, which are responsible for the stability to the structure. All amino acids in a polypeptide chain have L- configuration which can result in a stable helix if is right handed structure. Unlike ovalbumin proteins, casein proteins do not interact intramolecularly to from a tertiary structure. Local interactions may occur within the molecule in beta sheet but no interactions occur between the distant part of the same molecule. The molecule has a hydrophobic core. In β-Pleated, the conformation is linear a peptide chain and is stabilized by hydrogen bonding to adjacent parallel chains of the same kind. Steric interactions cause a slight bending of the peptide chains, resulting in a wrinkled distortion (the pleated sheet). Most proteins and large peptides such as ovalbumin have alpha-helix or beta-sheet segments, their tertiary structures may consist of less highly organized turns, strands and coils. Turns reverse the direction of the peptide chain, and are considered to be a third common secondary structure pattern. Beta sheets in ovalbumin are twisted by about 5 to 25° per residue; thus, the planes of the sheets are not parallel. B-pleated sheets that are formed by separate or single polypeptide chains and two neighbouring chains are held by intermolecular
hydrogen bonds, arranged in parallel ( same direction ) or anti parallel. The secondary structure of the protein is the result of interactions of side chains that are located within a few residues of each other. Proteins are sufficiently long that they can eventually fold back on themselves, allowing residues that are farther apart in the primary structure to interact with each other. These interactions give rise to the tertiary structure of the protein.
Fig.1.1 Secondary structure ( alpha helix)
In tertiary structure the protein fold is 3D.Moecules are very compact in this structure .Various amino acids interact (hydrophobic interaction or disulphide bonds) to form a cluster in the centre of the protein which completely folds and bends the protein to gives its required tertiary structure examples include collagen and ovalbumin protein.Ovalbumin is a globular nutrient and storage glycoprotein which plays a vital role in storage of amino acids around the body. Ovalbumin, for example, is the protein present in egg white ( 368 amino acids ), which is present 70% in egg white . The sequence includes six cysteines with a single disulfide bond between Cys74 and Cys121. The amino terminus of the protein is acetylated. Ovalbumin does not have a classical N-terminal leader sequence, although it is a secretory protein. Instead, the hydrophobic sequence may act as an internal signal sequence involved in transmembrane location. This protein is neutral and soluble in water. ( Huntington et al, 2001). Ovalbumin is rich in glutamate and aspartate amino acids. It has a tertiary structure and has a hydrophobic core in centre of the spherical shape (soluble in aqueous). The structure is small in length and width and therefore,
posses ovoid or spherical shape. Ovalbumin proteins are more complex in conformation than collagen proteins.
Collagen protein is a long fibre like structure ( parallel polypeptide) that contain three peptide chains which form triple helical structure by intramolecular hydrogen bonds which gives structures its stability. OH residues provide extra strength due to H-bonding, and the glyciene residues which are present in high amount( 33%) allow the protein to coil more tightly, since they fit on the inside of the helix. In a collagen fibre, three of these helices are coiled together to form a rope-like structure called a super helical coil. It is this structure that gives collagen its great strength. The secondary structure ( alpha helical conformation) further folds into a tertiary structure. The hydrophobic chains are held interiorly while the hydrophilic chains occur outside, which folds and coils the structure giving stability to the molecule. This tertiary structure is maintained by hydrogen bonds, disulphide bonds, ionic bonds, and hydrophobic interactions. This structure brings distant amino acids side chains closer. Covalent linkages are formed between lysine resides which strengthen the protein structure allowing it to stretch and recoil to original length when needed.(Woster, n.d).
Function of proteins.
Casein, is the milk phosphoprotein containing phosphoric acid esters of serine and threonine, these proteins are important for teeth and bones. Casein proteins are present in milk 70-80% and consist of a high number of proline peptides and have no disulfide bonds. Therefore, there it has moderately little secondary or tertiary structure. Casein is poorly soluble in water but readily dissolves in alkaline or strong acids, exhibiting hydrophobic properties, and appears in milk as a suspension of particles. Casein is also used in the manufacture of adhesives, binders, protective coatings, plastics (such as knife handles and knitting needles), fabrics and food additives and is non-toxic and highly stable. These proteins are rich in lysine which is one of the essential amino acids in milk. Casein proteins differ in different mammals milk such as animals and humans, and
are of three types; α, β and γ These differ one another in their molecular weights, rate of migration in an electric field and phosphorus content ( Horne, 2002; Wang et al , 2009).
Collagen is the most abundant protein in mammals, present in connective tissues (skin, bone, tendons, cartilage, the cornea, etc) which provides rigidity, protection, flexibility and lubrication. This protein is quite insoluble in water and affected by temperature change. Rich in glycine and alanine but also have serine and tyrosine. Contains three long polypeptide chains, each composed of about thousands of amino acids. Thhese chain curl into a regular triple helix, responsible for the elasticity of the skin and tendons .Under high heating these turn into gelation, gelation is soluble in water and gives a viscous solution, which can be used as a glue and as a thickener is food industry. If collagen is partially hydrolysed under mild condition ( acid, heating, etc), the three collagen strands separate into globular, random coils, producing gelatine which has lower antigenicity but still maintains some of its sequence to promote cell adhesion and proliferation ( Wang et al, 2009).
Many processes can lead to denaturation of protein which means it affects the secondary, tertiary or quaternary structure. Solubility is can result from the various levels of structure. For example, connective proteins are not soluble in water. While ovalbumin proteins are soluble in water and more sensitive to temperature or heating (unlike collagen , these lack glycine sequence) which means it can readily lose its secondary and tertiary structure, a process called denaturation (Huntington et al,2001). When denatured molecules aggregate to form protein network, the process is called coagulation or gel. This protein network formed traps water in food between its meshes to form gel or coagulation giving food its consistent ( egg meat) it is used for water absorbing (thickening) and to stabilise emulsion and foams.When an egg is cooked, the viscosity is reduced and the colour of egg changes, this color change indicates the change in structure that takes place in the albumin proteins during heating. The interactions of both polar and nonpolar units, often causes the protein to fold into spherical conformations. Although ovalbumin proteins are sensitive to denaturation (structural unfolding), some can be remarkably stable. Heating proteins can cleavage hydrogen bonds , ionic or hydrophobic bonds .Protein Structures are sensitive to temperature and PH specially the aggregation of proteins unfold alpha helix content is denatured or destroyed when heated under high temperature. Some proteins are tough and not soluble in water, either bonded covalently or positioned by other forces which are
not easily destroyed, examples of conjugated proteins include collagen ( Gossett et al,1984, Huntington et al, 2001).
The food proteins such as collagen, casein and ovalbumin provide high or low nutritional quality for humans. The quality of food depends on the structure and functional properties of protein. Some of functional properties of proteins are solubility, hydration, viscosity, texturing, formation of dough, emulsifying and foaming properties. Solubility is the ability of proteins to dissolve in water decreases during heat treatment, this loss of solubility has major consequences for emulsifying and foaming properties of protein.