mojahid عضو جديد
عدد المساهمات : 23 العمر : 38 تاريخ التسجيل : 20/08/2008
| موضوع: Amino Acids and Protein الثلاثاء 27 يناير - 3:43 | |
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Amino acids and proteins • Proteins are polymers of α-amino acids. • Amino acids are the building blocks of proteins. • The main carbon (The alpha carbon atom) is asymmetrical because the four groups linked to it are different. • Building Blocks - Amino Acids• An amino acid is defined as the molecule containing an amino group (NH2), a carboxyl group (COOH) and an R group. • It has the general formula:• R-CH(NH2)-COOH• The R group differs among various amino acids. • In a protein, the R group is also called a sidechain.• There are:• a -COOH, which is a carboxyl group (acidic). • a -NH2, which is an amino group (basic). • an -H hydrogen. • a residue R which varies depending on the amino acid.• All of the amino acids (except for proline) have a carboxyl group and an amino group. • At physiological pH the natural amino acids exist as zwitterions, with a negatively charged carboxyl group and a positively charged amino group.• Zwitterion is an amino acid with no ionizable R-group would be electrically neutral at this pH. • The configuration can be remembered as the CORN law. • Imagine looking along the H-Calpha bond with the H atom closest to you. • When read clockwise, the groups attached to the Calpha spell the word CORN (Richardson, 1981). • There are over 300 naturally occurring amino acids on earth, but the number of different amino acids in proteins is only 20. • Each amino acid has a similar, yet unique structure .• The side chains vary greatly in their complexity and properties. • The side chain of glycine is simply a hydrogen. • The side chain of tryptophan is based on the aromatic, bicyclic indole group. • All 20 different amino acids have the same structure, but their side chain groups (the R group) may vary in size, shape, charge, hydrophobicity, and reactivity. • The amino acids could be considered as the alphabet in which the proteins are written. • Peptides • The peptide is a chain of amino acids linked together by peptide bonds. • Polypeptides usually refer to long peptides whereas oligopeptides are short peptides (< 10 amino acids). • Proteins are made up of one or more polypeptides with more than 50 amino acids. • Peptide bond :• A peptide bond is the linkage between two amino acids, formed by the condensation reaction• Two amino acids can, under some circumstances, react together. • The result is the creation of a dipeptide. • A molecule of water is released in the process. This is the basic reaction involved in the synthesis of proteins• Primary structure• The primary structure of a protein refers to its amino acid sequence. • The amino acid in a peptide is also called a residue.The amino acid sequence (primary structure) of ribonuclease A (RNase A), which is an enzyme acting on RNA. Each letter represents an amino acid .• Secondary Structure • In a protein, certain domains may form specific structures such as α helix and β strand, which constitute the secondary structure of the protein.• Alpha helix• Beta strand, beta sheet and beta barrel• Motif• The secondary structure of RNase A, which contains three α helices and seven β strands. The α helix and b strand in RNase A. (a) Residues 1 to 36. (b) Residues 80 to 85. • Three-Dimensional Structure • The three-dimensional (3D) structure is also called the tertiary structure. • The ribbon representation of the 3D structure of RNase A. • Its atomic coordinates were obtained from Protein Data Bank • X-ray crystallography and nuclear magnetic resonance (NMR) are the major experimental techniques for determining the 3D structures of macromolecules. • Protein Data Bank is the most important site for structural information which can easily be accessed by the "PDB ID" of a macromolecule.• If a protein molecule consists of more than one polypeptide, it also has the quaternary structure, which specifies the relative positions among the polypeptides (subunits) in a protein.• The Twenty Amino Acids of Proteins :• Based on the physicochemical properties of R groups, the 20 amino acids of proteins may be classified as follows:• 1. Acidic: • including aspartic acid (aspartate) and glutamatic acid (glutamate). • In a neutral solution, the R group of an acidic amino acid may lose a proton and become negatively charged.• 2. Basic: • including lysine, arginine and histidine. • In a neutral solution, the R group of a basic amino acid may gain a proton and become positively charged. • Interaction between positive and negative R groups may form a salt bridge, which is an important stabilizing force in proteins.• 3. Aromatic: • including tyrosine, tryptophan and phenylalanine. • Their R groups contain an aromatic ring. • 4. Sulfur: • including cysteine and methionine. • Their R groups contain a sulfur atom (S). The disulfide bond formed between two cysteine residues provides a strong force for stabilizing the globular structure. • A unique feature about methionine is that the synthesis of all peptide chains starts from methionine . | |
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شيخ المختبر عضو مميز
عدد المساهمات : 236 تاريخ التسجيل : 11/09/2008
| موضوع: رد: Amino Acids and Protein الثلاثاء 27 يناير - 6:34 | |
| شكرا لك موضوع مميز
ننتظر منك المزيد
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hemato عضو فضي
عدد المساهمات : 546 تاريخ التسجيل : 13/08/2008
| موضوع: رد: Amino Acids and Protein الثلاثاء 27 يناير - 7:26 | |
| موضوع رائع أخي مجاهد
دائما متميز
في انتظار جديدك | |
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mojahid عضو جديد
عدد المساهمات : 23 العمر : 38 تاريخ التسجيل : 20/08/2008
| موضوع: رد: Amino Acids and Protein الجمعة 30 يناير - 22:44 | |
| يعطيكم العافيه... مع علمى التام بتقصيرى اتجاه المنتدى ولكن سنجتهد ان شاء الله | |
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