The Products of Protein Biosynthesis

The synthesis of protein is the ultimate result of the transcription and translation of the information contained in DNA. Various RNA species are formed via transcription (e.g., mRNA, tRNA, rRNA, etc.) and are then utilized in protein biosynthesis. Proteins can be classified in terms of function or in terms of their subcellular distribution in the cytoplasm, mitochondria, lysosomes, multienzyme complexes, brush border, nuclei, peroxisomes, etc. The nature of the specific tissues of the body is determined by the type and distribution of the various proteins of that tissue. Failure to produce the normal protein structure, sufficient amount of protein, or to regulate the production of protein in response to the appropriate stimulus results in disease in one or more tissues or organs, dependent on the distribution of that protein. The manifestations of the disease will depend on the function of the involved protein. These problems are considered in Chapter 15, The Biochemical Basis of Disease. 

Certain enzymes exist in both a reversibly membrane-bound state and a soluble state and, depending on intracellular conditions, will exist predominantly in one or the other state. While many enzymes show group specificity, using a number of related substrates, there is generally a preferred major physiological substrate that is metabolized 

Some proteins have primary functions that are nonenzymatic, yet under certain conditions they exhibit enzymatic activity (Table 9, F). Whether such enzymatic function is physiologically significant is not always clear. 

Hexokinase Cytoplasm Rat jejunal mitochondrial membrane (Anderson and King, 1975) 

Various enzymes Cytoplasm Skeletal muscle F-actin (Arnold and Pette, 1968, 1970) 

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The conformation of Xaa-Pro peptide bonds in the newly synthesized polypeptide chains prior to cellular folding is not known. The product of protein biosynthesis could be a uniform chain with all peptide bonds in the trans conformation. If this chain starts to fold immediately, then the trom-prolines would be in the correct conformation already, the cis-prolines would be in the incorrect isomeric state, and their trans — cis isomerization would be involved in the folding of all molecules. Alternatively, if there is sufficient time available for the Xaa-Pro bonds of the nascent chains to reach a cis/trans equilibrium (e.g., when folding is transiently arrested by binding to other proteins, such as heat-shock protein (HSP70), then the distribution of prolyl cis and trans isomers prior to cellular folding could be similar to the distribution found in the unfolded protein in vitro. Such a case was encountered in the maturation... 

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