Structures of Proteins. Primary structure - amino acid sequence

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1 Structures of Proteins Primary structure - amino acid sequence Secondary structure chain of covalently linked amino acids folds into regularly repeating structures. Secondary structure is the result of hydrogen bonding between the amide hydrogens and carbonyl oxygens of the peptide bonds. Two common types of secondary structure: Alpha helix Beta-pleated sheets Tertiary structure the polypeptide with its regions of secondary structure, ά-helix and β-pleated sheets, further folds on itself. The tertiary structures are maintained by the following structures. Hydrophobic interactions - These interactions group together in the interior of the protein, away from water, causing the polypeptide to fold. Van der Waals forces - These forces stabilize the close interactions between the hydrophobic residues. Hydrogen bonds - Chemical bonding between positively charged hydrogens and negatively charged atoms such as fluorine, oxygen, or nitrogen. Ionic bonds - These interactions occur between positively and negatively charged particles deep within the hemoglobin away from water. Covalent bonds between the thiol-containing amino acids. The soluble globular proteins have the 3 dimensional structures. Quaternary structure Many proteins are composed of aggregates of small globular peptides. The binding of the several polypeptides defines the quaternary structure of a protein. The same forces that holds the tertiary structure holds the quaternary structure. Some quaternary structure of a protein involves binding to a non-protein group. Example, many receptor proteins are glycoproteins. Each subunit of hemoglobin is bound to an iron containing heme group. The quaternary structure of hemoglobin consists of two identical ά-subunits and two identical β-subunits. 1

2 Hemoglobin 1. Structure protein consisting of 4 globin chains α 1, β 1, α 2, + and β 2. Each globin chain contains a small, rectangular heme group. In the center of each heme group is an iron (Fe) atom. The heme group is held in place by interactions with histidine side chains Iron atom forms two additional bonds on either side of heme plane. Only ferrous iron (ferrohemoglobin, Fe 2+ ) binds O 2. 2

3 Sickle Cell Blood Disorder Sickle Cell hemoglobin, HbS, differs from hemoglobin at the 6 th amino acid position of the beta chain where glutamic acid is replaced by valine. The sixth position in the normal beta chain has glutamic acid, while sickle beta chain has valine. Normal red cells retain their biconcave shape and move through the smallest vessels (capillaries) without problem. In contrast, the hemoglobin polymerizes in sickle red cells when they release oxygen. If only one of the beta globin genes is the "sickle" gene and the other is normal, the person is a carrier for sickle cell disease. The condition is called sickle cell trait. When sickle genes are inherited from both parents, sickle cell disease is manifested. The polymerization of hemoglobin deforms the red cells. The membranes of the cells are rigid due in part to repeated episodes of hemoglobin polymerization/depolymerization as the cells pick up and release oxygen in the circulation. These rigid cells fail to move through the small blood vessels, blocking local blood flow to a microscopic region of tissue. 3

4 Sickle cell trait partially protects people from the deadly consequences of malaria. Vitamin B12 Vitamin B12 (cobalamin) is only synthesised by micro-organisms. The core of the molecule is a corrin ring with various attached sidegroups. The ring consists of 4 pyrrole subunits, joined on opposite sides by a C-CH 3 methylene link, on one side by a C-H methylene link, and with the two of the pyrroles joined directly. Absorption and transport of Vitamin B12 Vitamin B12 binds a glycoprotein (intrinsic factor, I.F.) in stomach. I.F. is secreted by the parietal cells. Vitamin-intrinsic factor complex recognizes surface receptors of mucosal cells in ileum and is absorbed. It is transported around the body bound to specific a B12 binding protein (trans-cobalamin). It is stored mainly in the liver in amounts (3-5mg) sufficient to last a couple of years. 4

5 Functions of Vitamin B12 Vitamin B12 is used in the body in two forms: Methylcobalamin and 5- deoxyadenosyl cobalamin. The enzyme methionine synthase needs methylcobalamin as a cofactor. This enzyme is involved in the conversion of the amino acid homocysteine into methionine. Methionine in turn is required for DNA methylation. 5-Deoxyadenosyl cobalamin is a cofactor needed by the enzyme that converts L- methylmalonyl-coa to succinyl-coa. This conversion is an important step in the extraction of energy from proteins and fats. Furthermore, succinyl CoA is necessary for the production of hemoglobin. Procedure Safety Precaution; Wear gloves when handling hemoglobin solution. Discard ALL items used with the hemoglobin INTO THE BIOHAZARD WASTE CONTAINER. Column chromatography. Use column chromatography to separate hemoglobin (65000 daltons) and Vitamin B12 (1,350) daltons. Stationary phase is silica gel. Mobile phase is column buffer. Separation is by size. Vitamin B12 molecules are small and enters the gel molecule. The larger hemoglobin molecules are not absorbed by the gel and flow around the gel. The molecules are pushed out of the column by column buffer. Sources:

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