AP Syllabus focus:
‘Many signal transduction pathways alter proteins through phosphorylation cascades, in which sequential kinases activate one another.’
Cells often convert an external signal into an internal response by chemically modifying proteins. The most common modification is phosphorylation, which can rapidly switch protein activity and relay information through multi-step cascades.
Core idea: protein activity is regulated by modification
Protein phosphorylation as a reversible switch
Protein kinase: An enzyme that transfers a phosphate group from ATP to a specific amino acid on a target protein, changing the target’s activity, location, or interactions.
Phosphorylation typically targets serine, threonine, or tyrosine residues, altering protein shape and therefore function. Because phosphorylation is reversible, it is well-suited for tight regulation and rapid cellular decisions.
These diagrams show the chemical mechanism of phosphorylation (transfer of ATP’s terminal phosphate to a hydroxyl-bearing residue such as serine) and summarize reversibility by pairing kinases with phosphatases. Together, they connect the molecular chemistry of phosphate transfer to the functional idea of phosphorylation as a rapid, reversible regulatory switch in signaling. Source
Removing phosphate groups
Protein phosphatase: An enzyme that removes phosphate groups from phosphorylated proteins, often reversing the functional effect of a kinase.
Kinases and phosphatases act as opposing controls; the balance between them determines how much of a given protein is in the “on” versus “off” state at any time.
Phosphorylation cascades: sequential kinases activate one another
What a cascade is
A phosphorylation cascade is a stepwise pathway in which one activated kinase phosphorylates (and activates or inhibits) the next protein, commonly another kinase.
This diagram illustrates a MAPK signaling pathway as a multi-kinase phosphorylation cascade, where an upstream kinase activates downstream kinases in sequence. It emphasizes how information can be relayed and amplified through successive phosphorylation steps, ultimately affecting target proteins that drive cellular responses. Source
This directly matches the syllabus emphasis on sequential kinases activating one another.
Typical flow (conceptual)
A signalling event produces an activated kinase (often via a prior modification or binding event).
Kinase 1 phosphorylates Kinase 2, changing Kinase 2’s activity.
Kinase 2 phosphorylates Kinase 3, continuing the relay.
The final kinase phosphorylates effector proteins that execute the cellular change (for this subsubtopic, focus is the modification itself, not the broader response types).
Why cascades are common
Signal propagation: a chemical message is passed forward without the original signal molecule entering all cellular compartments.
Multiple control points: each step can be regulated by phosphatases, inhibitors, or protein availability.
Specificity: kinases recognise particular target sequences, limiting inappropriate phosphorylation.
Functional consequences of phosphorylation (protein modifications)
Changing enzyme activity
Phosphorylation can:
Activate enzymes by stabilising an active conformation.
Inhibit enzymes by blocking an active site or preventing required conformational change.
Modifying protein-protein interactions
Adding a phosphate can create or destroy binding sites, reshaping signalling complexes:
Promotes assembly of multi-protein complexes needed for downstream phosphorylation.
Prevents interactions that would otherwise keep a pathway inactive.
Altering localisation
Phosphorylation may expose or hide localisation signals, shifting where a protein functions:
Cytosol ↔ membrane association
Cytosol ↔ nucleus movement (as controlled by phosphorylation-dependent transport signals)
Affecting protein stability
Some phosphorylation patterns mark proteins for:
Stabilisation (longer functional lifetime)
Degradation (shorter lifetime), by recruiting proteins that target them to breakdown pathways
Regulation and termination within phosphorylation-based signalling
Turning signals off
Pathways end when:
Phosphatases dephosphorylate key kinases or effectors.
Kinases become inactive due to loss of activating phosphorylation.
Competing proteins sequester kinases away from their targets.
Avoiding runaway signalling
Cells prevent inappropriate cascade activity by:
Keeping kinases in inactive conformations until needed.
Using phosphatases to reset proteins quickly.
Restricting kinases to particular locations so they only meet correct targets.
FAQ
Scaffold proteins bind several kinases at once, holding them close together.
This can:
speed up phosphorylation by increasing effective concentration
reduce accidental cross-talk by isolating pathway components
They phosphorylate different amino acids: serine/threonine versus tyrosine.
Tyrosine phosphorylation is often rarer but strongly regulatory, frequently creating docking sites for proteins with phosphate-binding domains.
Common approaches include phospho-specific antibodies and protein separation methods that shift with phosphorylation state.
Mass spectrometry can identify exact modified residues and quantify changes across many proteins at once.
Yes. Many bacteria and some plant pathways use multi-step phospho-relays, where phosphate is transferred between different proteins in sequence.
These relays still use reversible phosphorylation logic but rely on different enzyme families than many animal cascades.
Many inhibitors compete with ATP binding or block the kinase active site.
Resistance can occur if mutations alter the binding pocket, if cells upregulate alternative kinases, or if pathway wiring changes to bypass the inhibited step.
Practice Questions
Explain how phosphorylation can change the activity of a target protein. (2 marks)
Phosphate group is added by a kinase to the protein (1).
This alters protein shape/charge and therefore switches activity or interactions on/off (1).
Describe how a phosphorylation cascade transmits and regulates a signal using sequential kinases. (5 marks)
A kinase is activated and phosphorylates a second protein kinase (1).
The second kinase becomes activated (or inhibited) and phosphorylates a third component, often another kinase (1).
The sequence continues, relaying information via phosphorylation at multiple steps (1).
Phosphatases remove phosphates to reverse effects and terminate/reset the pathway (1).
Multiple steps provide regulatory control points and maintain specificity through kinase-target recognition (1).
