Ribosome-bound enzyme

T3 and T7 Lysozyme. T3 and T7 lysozyme synthesis under the direction of the corresponding DNAs was performed in a DEAE system. The product consists of free enzyme and most probably ribosome-bound enzyme based... 

The endoplasmic reticulum (ER) is an extensive closed membrane system consisting of tubular and saccular structures. In the area of the nucleus, the ER turns into the external nuclear membrane. Morphologically, a distinction is made between the rough ER (rER) and the smooth ER (sER). Large numbers of ribosomes are found on the membranes of the rER, which are lacking on the sER. On the other hand, the sER is rich in membrane-bound enzymes, which catalyze partial reactions in the lipid metabolism as well as biotransformations. 

Much of the chemistry of the cell is common to all living systems and is directed towards ensuring growth and cell multiplication, or at least the survival of the cell. Organisms also share various structural characteristics. They all contain genetic material (DNA), membranes (the boundary material between the cell and the environment), cytoplasm (small particulate materials, ribosomes and enzyme complexes), and cell walls or surfaces (complex structures external to the membrane). In addition, there are various distinct membrane-bound organelles in eukaryotic organisms which have specialised functions within the cell (Tables S.4, 5.5 and 5.6)(8-, 7). 

Studies in this laboratory for the past several years have been concerned with the elucidation of the latter steps in this series of reactions. Specifically, efforts have been directed toward the characterization of the reaction involving the transfer of aminoacyl sRNA to mammalian ribonucleoprotein particles, the enzymatic and cofactor requirements, and possible intermediates in this process. The evidence obtained indicates that aminoacyl transfer is an enzymatic reaction requiring at least two enzyme fractions, which have been resolved and partially purified, GTP and a sulfhydryl compound further, the possibility exists that a ribosome-bound sRNA-amino acid (or peptide) compound is formed as an intermediate in this reaction. 

An unexpected finding was that phosphati-dylserine S3mthase of E. coli is tightly bound to ribosomes. This enzyme, which incorporates serine into phospholipids according to step h of Fig. 21-4, is responsible for synthesis of fhe principal membrane... 

Up to now, we have simply described noncovalent, nonenzymic binding of messenger RNA or synthetic polynucleotide and have said nothing about its relation to binding and peptide bond formation. Such a relationship can best be understood by describing the results of experiments in which poly U was incubated with reticulocyte ribosomes. When poly U is added to washed ribosomes in the presence of acyl phenylal-anyl tRNA, one molecule of phenylalanyl tRNA is attached per ribosome, but no peptide bonds are formed. If an enzymic preparation partially purified from the cell sap and GTP are added, GDP is formed, and a second phenylalanyl tRNA molecule is bound to the ribosome. Another enzymic preparation must be added to the incubation mixture for peptide bond formation. (We shall see later that the catalytic molecules involved in these reactions have been identified.)... 

Lysosomal enzymes, including many hydrolases responsible for degrading cellular waste products are synthesized on ER-bound ribosomes. 

The internal volume bounded by the plasma membrane, the cytoplasm (Fig. 1-3), is composed of an aqueous solution, the cytosol, and a variety of suspended particles with specific functions. The cytosol is a highly concentrated solution containing enzymes and the RNA molecules that encode them the components (amino acids and nucleotides) from which these macromolecules are assembled hundreds of small organic molecules called metabolites, intermediates in biosynthetic and degradative pathways coenzymes, compounds essential to many enzyme-catalyzed reactions inorganic ions and ribosomes, small particles (composed of protein and RNA molecules) that are the sites of protein synthesis. 

However, little is currently known about its synthesis. The protein component may be assembled on the ribosomes by the normal mechanism of protein synthesis.324 L-Proline is known to be the precursor of the hydroxy-L-proline found in the glycoprotein,324,325 hydroxylation of the peptide-bound L-proline being catalyzed, in carrot cells, by cytoplasmic enzymes.324... 

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