Post-translational Modification and Protein Turnover
Most proteins are altered after they are made: phosphate groups, sugars, lipid tails and cleaved segments all change what a protein does. Tagging with ubiquitin sends it to the proteasome, so the amount of a protein is set by degradation as much as by synthesis.
What a learner can do afterwards
- Names a modification and says what it changes: activity, location, binding partners or lifetime.
- Explains why a cell with unchanged transcription can still change its protein levels.
- Reads a half-life difference between two proteins as a statement about tagging and degradation.
1 · Read
Most proteins are altered after they are made. Enzymes clip chains and add tags: phosphate groups, sugars, lipid tails, and address labels. In phosphorylation, a phosphate from ATP is attached, usually flipping the protein between on and off states. One small tag can twist a whole active site open or shut.
Each edit can flip activity on or off, move the protein, change its binding partners, or mark it for a short life. This lets a cell react in seconds without transcribing a single new message, since the proteins wait on site for their tags. Waves of phosphorylation drive the cell cycle from one phase to the next. Gene regulation sets the cast, but post-translational edits direct the play.
Tagging with ubiquitin sends a protein to the proteasome, where old or damaged chains are shredded. If destruction speeds up while production holds steady, the level falls even though transcription never changed. Read a short half-life as a statement about tagging: the protein is marked for quick recycling. The cell is a bathtub with tap and drain both open, and the water level answers to both.
When levels shift with steady mRNA, suspect the drain before the tap. The same chain can serve as an active enzyme one moment and tagged trash the next.
Tags flip, move, or doom proteins, so levels answer to destruction as much as synthesis.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.