Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Metabolism of Nitrogen-Containing Compounds
Incorporation of NH3 into Amino Acids and Proteins
Nitrogen Turnover in Cells

The reactions by which Amino Acids are incorporated into Proteins were briefly reviewed in Chapter 11 (Section D, 1) and will be discussed further in Chapter 15 (Section B). However, it should be kept in mind that The formation of biologically active catalysts, Hormones, and structural proteins is frequently not complete once the peptide chain leaves The Ribosome and folds into a specific preferred conformation. Very often, proteins subsequently undergo site-specific Hydrolysis and various covalent modifications, as described in Chapter 11 (Section D, 2).

Some chemical remodeling (Processing) of newly formed Peptides likely takes place already in the Cytoplasm [29], but a portion of it occurs after the secreted proteins are segregated within the cisternae (microsomal cavities) of The Endoplasmic reticulum [30]. It is believed that the Ribosomes synthesizing these proteins are located on the cytoplasmic side of the endoplasmic reticulum membrane, and that the newly formed peptide chains are translocated across the membrane into these cisternae. Various modifying Enzymes may act there.

One of the unexpected and remarkable discoveries made during Schoenheimer's research (Section B) was that cellular proteins exist in a steady state of continuous Synthesis and degradation. Thus, the pathways of synthesis and hydrolysis form a metabolic loop (Chapter 11, Section A, 1). A relevant generalization is that proteins secreted into the extracellular fluid often exhibit a faster turnover rate than proteins that remain intracellular. At the same time, within Cells, certain proteins degrade significantly faster than others, which is of great importance for Metabolic control mechanisms. In plants, relatively low protein turnover rates predominate.

While some proteinases that cleave intracellular proteins are presumably located in the cytoplasm, cathepsins with an acidic pH optimum reside in Lysosomes [31, 32], and a neutral proteinase has been found in Peroxisomes. Cathepsins, as well as collagenases and elastases, appear to be secreted into the extracellular space, where they participate in The breakdown of Connective Tissue. Excessive activity of these enzymes may play a crucial role in certain pathologies [33].