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Haemoglobin is a transport protein that carries oxygen from the lungs to the tissues and carries carbon dioxide from the tissues back to the lungs. In order to function most efficiently, haemoglobin needs to bind to oxygen tightly when the oxygen concentration is high in the lungs and be able to release oxygen rapidly in the relatively low partial pressure in the tissues. I will be comparing the structure and function of haemoglobin with the structure and function of collagen, which is a structural protein.
Collagen’s functions are quite different from those of globular proteins such as enzymes; tough bundles of collagen called collagen fibers are a major component of the extracellular matrix that supports most tissues and gives cells structure from the outside. Collagen is also present in certain cells as it has great tensile strength, and is the main component of fascia, cartilage, ligaments, tendons, bone and teeth.
Primary Structure; the number, type and sequence of amino acids
The primary level of structure in a protein is the linear sequence of amino acids, formed by a condensation reaction.
The primary structure of these proteins are very different. The haemoglobin molecule consists of four polypeptide (globin) chains, where as collagen is made up of three polypeptide chains wound round each other. In Haemoglobin there are a wide range of amino acid constituents in the primary structure, in contrast 35% of collagen’s primary structure is glycine. This difference in primary structure causes the proteins themselves to be different; due to the original different primary structure the haemoglobin protein is soluble in water whereas the collagen protein is not.
Secondary structure; Formed when the chain of amino acid coils or folds to form an alpha helix or beta pleated sheet. Haemoglobin is comprised of four polypeptide subunits, two with alpha helix secondary structure and two with beta pleated sheet form. All four components carry a heme group that can bind to oxygen, and all four components must be present to form haemoglobin. The shape of the haemaglobin affects its ability to carry oxygen, and travel freely throughout the circulatory system. Whereas Collagen’s secondary structure involves three strands of protein bonded together between the chains with hydrogen bonds and then twisted into a helix that is referred to as a ‘collagen helix’.
Tertiary structure; the final three dimensional shape of protein is formed when these coils and pleats coil or fold. Due to interactions between R-groups of the different amino acids. The tertiary structure or overall shape of collagen protein subunits is a helical chain. Due to the ways in which the amino acids are arranged the protein can be hydrophobic or hydrophilic. Soluble proteins, like haemoglobin will fold with the hydrophobic side on the inside and the hydrophilic side on the outside. As with all proteins the tertiary structure of each subunit is held in place by a number of bonds and interactions, these interation give the subunits and whole molecule very specific shapes, this is why haemoglobin and collagen differ so greatly in structure and in function as the shape of the molecule (due to the tertiary structure) is vital for the molecule to carry out its function.
Quaternary structure; some proteins are made up of more than one polypeptide subunit joined together. The quaternary structure of haemoglobin consists of four polypeptide subunits. Two are called alpha chains and two are called beta chains. The four subunits together form one haemoglobin molecule, which is water soluble. In comparison, The quaternary structure of collagen consists of three left-handed helices twisted into a right-handed coil. Fibrous proteins, like collagen contain polypeptide chains arranged in long strands or sheets, whereas globular proteins, like haemoglobin have polypeptide chains that are folded in a spherical shape.
Due to the differences in structure of collagen and haemoglobin their shape and function also differ greatly. The triple helix structure of collagen gives the structure strength and therefore the function of collagen is to provide mechanical strength in many areas. Where as the presence of a prosthetic group, Haem, in the structure of haemoglobin means oxygen can bind to the iron.
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