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Hemoglobin is a protein, which is one of the four basic biological molecules of life. Proteins have many functions and are versatile molecules. They are important for building and repairing cells and tissues and also as enzymes that are used in catabolic processes within an organism. Hemoglobin is the main substance of the red blood cell and it helps red blood cells carry oxygen from the air in our lungs to all the parts of our body. The other three molecules of life are carbohydrates, lipids, and nucleic acids. Proteins can also be called polypeptide chains because they are made up of subunits, amino acids that are linked with peptide bonds. Since hemoglobin has such an importance to the function in our bodies it has an importance in our society as well. In order to understand how and why hemoglobin is essential one must understand the chemistry behind this molecule.
There are thousands of different polypeptides due to the variety of amino acid arrangements in forming chains. Amino acids are organic compounds that have an amino group, a hydrogen atom, and a carboxyl group attached to an asymmetric carbon (a carbon that has four different groups attached to it with covalent bonds). In all amino acids these three groups are always present. It is the fourth group that always differs and makes all of the different amino acid structures. This group is known as the R group because it changes for every amino acid. There are 20 basic amino acids that are found in proteins the simplest being glycine (R group = H). See Figure 1 for a view of the amino acid structure. The number and arrangement of these amino acids is what determines different polypeptides. The carboxyl group of one amino acid bonds with the amino group of another amino acid to form a polypeptide with a peptide bond.
Every amino acid can be classified as either polar or non-polar and consequently hydrophobic or hydrophilic. The R groups of the amino acids determine the polarity of the complete amino acid. If a molecule is known to be Polar it has a slightly negative or positive end because one of the atoms draws more electrons than the others. Polar molecules are attracted to water because water is also polar (polar molecules have charges, and the negative charge of one attracts the positive charge of the other because opposites attract) with oxygen having a slightly negative charge and both hydrogen atoms consiting of a slightly positive charge. Therefore, polar molecules are hydrophilic (like to be affiliated with water) and nonpolar molecules are hydrophobic (hate water). See Figure 2 for a look at a polar amino acid and view Figure 3 for a nonpolar amino acid. Some amino acids are also dipolar or charged.
Proteins take on many classifications in order to differentiate them from other proteins. Some of these classifications are determined based on their structure. One such classification is whether a protein is globular or fibrous. Hemoglobin is a globular protein. Another classification based on structure has four categories. Proteins undertake four structures in order to get to their final structure. They are primary, secondary, tertiary, and quaternary. This classification is based on whether there is a straight amino acid chain, if the polypeptide chain is coiled or in a pleated sheet (fan-like), if the coiled chain is twisted and made into a globular structure, or if two or more coiled and twisted chains are put together. The difference of proteins results in the differences in these structures. Many proteins only go through three of these steps because they don't mix with other globular structures. Some proteins, including hemoglobin, form quaternary structure when 2 or more polypeptide sequences come together and wrap around each other. In a hemoglobin molecule the polypeptide subunits are called globins. When four globins and four heme groups come together, hemoglobin is formed. Hemoglobin goes through all four steps and it is known to take on the quaternary structure, the most complex of the structures.
The primary structure in the construction of a protein is the linking of amino acids into polypeptide chains. See Figure 4 for the primary structure of hemoglobin. The hemoglobin molecule is made up of four polypeptide chains, Alpha 1, Beta 1, Alpha 2, and Beta 2, noncovalently constrained to each other. It is essential to realize that Alpha and Beta refers to the two types of chains rather than their secondary structure. In fact, both types of polypeptide chains are in the form of alpha helix. Both the Alpha 1 chain and the Alpha 2 chain are matching and they each contain 141 amino acids. A change in even one of these amino acids affects the whole protein greatly. Furthermore, any change in the primary, secondary, tertiary, or quaternary structure, could change the whole structure, and as a result the whole function of the protein. Bo
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hemoglobin, amino acid,
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