Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation

The Mechanism of METABOLISM/35.html">Protein Biosynthesis, with all its diverse biological activity and species Specificity, has been one of the major challenges in The history of biochemistry. For many years, even very basic questions regarding Protein Synthesis remained unanswered. For instance, are Proteins formed as a single entity all at once, or are they assembled from numerous short, pre-synthesized Peptides? Or consider another question: could all cellular proteins be derived from a single long precursor polypeptide through specific modifications of its side (R) groups? Prior to the early 1950s, it was not even reliably established that proteins are distinct chemical compounds with a defined molecular weight, a specific Amino Acid Composition, and a precise sequence of amino acid residues.

Today we already know a great deal about The process of protein synthesis, yet it is quite possible that this is merely a fraction of what remains to be discovered. In all likelihood, protein synthesis represents the most complex of all biosynthetic processes, requiring a vast array of Enzymes and other specialized macromolecules. In Eukaryotic Cells, protein synthesis involves over 70 different ribosomal proteins, at least 20 enzymes required for the activation of amino acid precursors, over a dozen auxiliary enzymes and other specific protein factors for initiation, elongation, and termination of polypeptide synthesis, as well as perhaps no less than 100 additional enzymes participating in protein Processing (maturation). When we add to this list over 70 types of transfer and Ribosomal RNAs, it becomes clear that polypeptide synthesis requires the coordinated action of nearly three hundred different macromolecules. Furthermore, many of these macromolecules are organized into complex three-dimensional ribosomal structures, where mRNA undergoes translocation as the polypeptide chain is synthesized.

Despite the immense complexity of the protein biosynthesis machinery, polypeptide formation proceeds at an extraordinarily high rate. For example, a ribosome in E. coli takes just five seconds to synthesize a polypeptide chain consisting of one hundred amino acid residues. Moreover, as we will see below, the synthesis of thousands of different proteins within each Cell is strictly regulated, ensuring that under specific metabolic conditions, precisely the required number of molecules for each protein is produced.