Medical brain notes 25

  Protein Metabolism Introduction The biosynthesis of protein molecules in the cell by sequential addition of various amino acids using peptide bond is called protein synthesis. The amino acids are linked together in succession to produce a linear polypeptide chain. The polypeptide chain is a unit of a protein molecule.

  Protein biosynthesis In a protein molecule amino acids are joined together by peptide bonds. In the process of protein synthesis also known as translation of mRNA, the amino acids are added sequentially in a specific number. The protein synthesizing mechanism involves the following steps: Transcription The formation of RNA complementary to a DNA strand is called transcription. In this process, the RNAs required for protein synthesis are synthesized on DNA strands. This reaction is catalyzed by the enzyme RNA polymerase.   The enzyme, RNA polymerase I, II, III are involved in the synthesis of rRNA (ribosomal RNA) mRNA (messenger RNA) and tRNA (transfer RNA) respectively in the eukaryotes. In prokaryotes only one type of RNA polymerase is present to synthesize all the three classes of RNA.   In the DNA double helix, one of the strands serves as a template to produce RNA. The RNA produced by transcription is inactive and is called pre-RNA. They become active after further ...

  Translation Translation is a process by which the base sequence of DNA transcribed to the mRNA is interpreted into amino acid sequence of a polypeptide chain.   Translation involves the following steps:   1.             Activation of amino acid   2.             Transfer of activated amino acid to tRNA   3.             Initiation of polypeptide chain   4.             Elongation of polypeptide chain   5.             Termination of polypeptide chain   1.  Activation of amino acid Amino acids, the building blocks of proteins, are present in the cytoplasm. They are activated before they are transported by tRNA. The amino acids are activated by ATP with the help of the enzyme ...

  Metabolism of Proteins The ingested proteins are metabolised to amino acids by peptide bond cleaving enzymes known as proteinases.   1.  General reactions of amino acids The general reactions of amino acids include deamination, transamination and decarboxylation. The reactions of deamination and transamination bring about the formation of keto acids which can undergo a further series of changes. Inter-conversion between keto acids and amino acids results in the synthesis of many nutritionally non essential amino acids. These provide for the synthesis of protein and important non-protein nitrogenous materials. During protein synthesis the amino acids are absorbed from the blood, as the liver does not store them.   2.  Catabolism of amino acids Although each amino acid follows its own specific metabolic pathway, a few general reactions are found to be common in the catabolism of nearly all the amino acids. Most of the amino acids are converted to a-keto acids by...

  Urea Cycle   Living organisms excrete the excess nitrogen resulting from the metabolic breakdown of amino acids in one of three ways. Many aquatic animals simply excrete ammonia. Where water is less, plentiful processes have evolved that convert ammonia to less toxic waste products which require less water for excretion. One such product is urea, which is excreted by most terrestrial vertebrates; another is uric acid, which is excreted by birds and terrestrial reptiles. Accordingly, living organisms are classified as being either ammonotelic (ammonia excreting), urotelic (urea excreting) and uricotelic (uric acid excreting). Some animals can shift from ammonotelism to urotelism or uricotelism if their water supply becomes restricted. Urea is synthesised in the liver by the enzymes of the urea cycle. It is then secreted into the blood stream and sequestered by the kidneys for excretion in the urine.The urea cycle reactions were elucidated by Hans Krebs and Kurt Henseleit. Thi...