Biochemistry - Chemical Reactions in Living Cells Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Metabolic Loops and Biosynthetic Families
Metabolic Loops

Together, the pathways of Biosynthesis and biological degradation form continuous loops—series of reactions that occur simultaneously and often within the same cellular compartment. Metabolic loops frequently originate in the central regions of Carbohydrate METABOLISM pathways, involving the transformation of three- and four-carbon compounds (phosphoglycerates, Pyruvate, or oxaloacetate). After the loss of a certain number of carbon atoms as CO₂, the remaining portion of the compound re-enters the main metabolic stream by joining the primary catabolic pathway that leads to acetyl-CoA and oxidation in The Tricarboxylic Acid Cycle. However, many other variations may also occur. Not all loops are closed cycles within a given species; humans, for instance, are unable to synthesize Vitamins and Essential Amino Acids. Consequently, from the standpoint of synthesizing these compounds, we depend on other organisms, yet we are capable of breaking them down and assimilating them. Certain metabolites (such as uric acid) are excreted by humans and subsequently catabolized by Bacteria. From a chemical perspective, nature as a whole can be viewed as an extremely complex system of branched and interconnected metabolic cycles. Thus, all biosynthetic pathways operating in autotrophs represent specific segments of metabolic loops that culminate in oxidation, leading back to CO₂.

An important characteristic of metabolic loops is that it is often impossible to determine precisely where biosynthesis ends and biological degradation begins. The end product X required by a Cell may serve as a precursor for another cellular component Y, which must undergo degradation to complete the loop. The reactions in which compound X is converted into compound Y can be viewed both as a biosynthetic reaction (for Y) and as a degradation reaction (for X).