Basic Principles of Regulation and General Pathway
The control of purine production is carried out through allosteric inhibition. This is a classic example of negative feedback (or retroinhibition), in which accumulating end products of a metabolic pathway suppress the activity of enzymes located at its very beginning. Such products include inosine monophosphate (IMP), adenosine monophosphate (AMP), guanosine monophosphate (GMP), and their di- and triphosphate derivatives.
Regulation can be conditionally divided into several levels. The very first level is the control of early stages, which represent the common pathway of synthesis and determine the overall rate of the process. Two key regulatory enzymes operate here:
- PRPP synthetase catalyzes the first reaction: the interaction of ribose-5-phosphate and ATP to form 5-phosphoribosyl-1-pyrophosphate (PRPP). This enzyme is activated by inorganic phosphate and inhibited via feedback by excess AMP, GMP, and IMP.
- PRPP amidotransferase (also known as glutamine-PRPP amidotransferase) is responsible for the rate-limiting step of the entire pathway—the transfer of an amide group from the amino acid glutamine to the PRPP molecule. This results in the formation of 5-phosphoribosyl-1-amine. The substrate of the reaction (PRPP) acts as a potent activator of the enzyme, whereas the end products (AMP, GMP, IMP) exert allosteric inhibition.
An essential feature of the common pathway control is the synergistic effect. Inhibition of the initial reactions is maximally effective only with a simultaneous increase in the concentration of both adenylic and guanylic nucleotides in the cell.
Branch Point and Specific Inhibition
Upon completion of the common pathway, a key intermediate metabolite is formed: inosine monophosphate (IMP). At this stage, the metabolic pathway branches into two: one branch leads to the formation of adenylic nucleotides, and the other to guanylic nucleotides. The cell must maintain a strict balance between these two flows, so additional control mechanisms are engaged at the branch point.
Enzyme activity in each of the branches is specifically reduced in the presence of an excess of the corresponding end product:
- AMP synthesis branch: The conversion of IMP to adenylosuccinate is catalyzed by the enzyme adenylosuccinate synthetase. This enzyme is specifically inhibited by the product of its own branch—AMP molecules.
- GMP synthesis branch: The conversion of IMP to xanthosine 5'-monophosphate (XMP) is provided by the enzyme IMP dehydrogenase. The activity of this enzyme is suppressed by the excessive accumulation of GMP.
Cross-Regulation and Energy Balance
In addition to direct allosteric inhibition, there is an elegant mechanism ensuring the balanced content of adenylic and guanylic nucleotides. It is based on the cross-dependency of synthesis on high-energy compounds.
- To synthesize AMP from IMP, the cell requires energy in the form of GTP molecules.
- For the synthesis of GMP from the intermediate metabolite XMP, the expenditure of ATP energy molecules is required.
- The biological significance of this cross-regulation is immense. It prevents a distortion in the nucleotide pool. If an excess of ATP accumulates in the cell, it inevitably stimulates the synthesis of guanine derivatives (GMP). Conversely, the accumulation of GTP accelerates the production of adenylic nucleotides. If a deficit of one of these nucleotides arises, the synthesis of the other is automatically inhibited, avoiding a dangerous imbalance when preparing the cell for division or active RNA synthesis.
Tissue Specificity of Synthesis
It is important to note that not all cells in the body possess the full complement of enzymes for de novo purine ring synthesis. Some tissues are completely or partially devoid of this capability.
First of all, the following are incapable of purine synthesis:
- Mature erythrocytes.
- Polymorphonuclear leukocytes.
- Brain tissues (capable only partially).
Since nucleotides are vital for any cell for energy metabolism and numerous other functions, these tissues depend on an external supply of purines. The main organ ensuring their synthesis and subsequent export is the liver. From the liver, preformed purine bases and nucleosides are transported via the blood to the cells in need, where they are utilized by specialized salvage pathway enzymes.