Biological significance of the process
Phosphorylation and dephosphorylation represent a crucial way of controlling metabolic pathways. The main task of this mechanism is to ensure rapid regulation of key enzymes.
The process is triggered depending on changing environmental conditions. The main trigger for such changes is hormonal regulation. Hormones transmit a signal that is transformed within the cell into a command to modify the required enzymes, thereby restructuring the entire metabolism to meet the current needs of the organism.
Mechanism of phosphorylation
Phosphorylation is the process of covalent attachment of a phosphoric acid residue to an enzyme molecule.
- Modification substrate: the reaction does not occur at any site of the protein, but strictly at the OH groups (hydroxyl groups) of specific amino acid residues making up the enzyme.
- Catalytic enzymes: the reaction is carried out by specialized enzymes called protein kinases.
- Phosphate source: the phosphoric acid residue donor in this reaction is always an ATP molecule.
Mechanism of dephosphorylation
Dephosphorylation is the reverse process in which the previously attached phosphate group is removed, returning the enzyme to its original basal state.
- Catalytic enzymes: other specific enzymes—phosphoprotein phosphatases—are responsible for the removal of the phosphate.
- Chemical nature: the process is a hydrolysis reaction, during which the phosphoric acid residue is cleaved from the OH group of the enzyme's amino acid residue.
Effect on enzyme structure and activity
How exactly does the addition of a small phosphate group change the function of a huge protein molecule? The whole secret lies in the spatial structure.
- Conformational change. The attachment of the phosphate group inevitably causes a change in the spatial configuration (conformation) of the enzyme's active site.
- Change in affinity. Due to the new shape of the active site, the affinity of the enzyme for its substrate changes (it begins to bind to it more easily or, conversely, worse).
- Dual result. It is important to understand that phosphorylation is not equivalent to unconditional activation. The effect is strictly individual for each enzyme:
- Some enzymes are activated upon phosphorylation.
- Other enzymes are inhibited (their activity drops) upon phosphorylation.
Subsequent dephosphorylation always produces the opposite effect, completely returning the enzyme to its original structural and functional state.
Briefly: answers to topical questions
Which specific amino acid residues undergo phosphorylation?
Hydroxyl (OH) groups of specific amino acids in the protein structure undergo phosphorylation. These amino acids include:
- Serine — binds to phosphoric acid via an ester bond.
- Threonine — is phosphorylated by various protein kinases.
- Tyrosine — undergoes phosphorylation by tyrosine protein kinases (for example, during transautophosphorylation of growth factor and insulin catalytic receptors).
Which hormones trigger the cascade of enzyme phosphorylation in the cell?
The cascade of intracellular enzyme phosphorylation is triggered by hormones acting during energy deficiency or stress. These include:
- Glucagon — acts predominantly in the liver during starvation, triggering the adenylate cyclase cascade.
- Adrenaline — acts in muscles during physical work.
These hormones increase cAMP levels and activate protein kinase A, which transfers the phosphate group to target enzymes.
Give examples of key metabolic enzymes that are activated upon phosphorylation.
The attachment of a phosphate group converts a number of metabolic enzymes into an active state. These enzymes include:
- Glycogen phosphorylase — a key enzyme of glycogen breakdown, active in its phosphorylated form.
- Phosphorylase kinase — is converted into the active form by protein kinase A for the subsequent activation of glycogen phosphorylase.
- Tyrosine hydroxylase — is activated by phosphorylation (the Michaelis constant and affinity for the inhibitor noradrenaline decrease).
Which second messengers participate in the activation of protein kinases?
The following second messengers participate in intracellular signal transduction and the activation of specific protein kinases:
- cAMP — binds to regulatory subunits and converts protein kinase A into the active form.
- cGMP — reversibly binds to the regulatory centers of protein kinase G (4 molecules are required for activation).
- Calcium ions ($Ca^{2+}$) — together with diacylglycerol activate protein kinase C, and in a complex with calmodulin — calmodulin-dependent protein kinases.
Protein KINASE — "throws" (kinetikos) phosphate onto the enzyme (taking it from ATP). Phosphoprotein PHOSPHATASE — tears off PHOSPHATE (via hydrolysis).
Class and exam questions
Which enzymes carry out phosphorylation and where do they get the phosphate from?
Phosphorylation is carried out by protein kinase enzymes. They use an ATP molecule as a donor of the phosphoric acid residue.
To which exact parts of the enzyme does the phosphate group attach?
The phosphate group attaches to the OH groups (hydroxyl groups) of specific amino acid residues in the enzyme structure.
Does phosphorylation always lead to enzyme activation?
No, the effect can be dual. Due to changes in the conformation of the active site, some enzymes are activated, while others are inhibited (become less active).
What else to study on this topic
- Structure and conformation of the enzyme active site
- Enzyme affinity for substrate
- Mechanisms of hormonal regulation of metabolism
- Chemical structure and role of the ATP molecule
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