Question

Which of the following best describes how ubiquitin terminates Cdk complex activity?

Group of answer choices

Ubiquitin promotes transcription of Cdk complex inhibitors

Ubiquitin chains are added to the cyclin, directing it to the proteasome for degradation

Ubiquitin activates membrane pumps that re-establish Ca²⁺ gradients

Ubiquitin binds to the Cdk active site and inhibits its enzyme activity

Answer 1

Ubiquitin is a protein that is known to lead to protein degradation by attaching to lysine residues in the protein via post-translational modifications. Cdk or cyclin-dependent kinases are regulators of cell cycle which are activated by binding of cyclins and deactivated when the cyclins are removed. As such the Cdk levels in cells are constant but cyclin levels vary since their levels are kept under control by proteolysis signaled via ubiquitination of cyclins.

All regulation done by ubiquitin is through marking a protein for proteolysis. it could be of transcription factors leading to transcriptional repression or repressor proteins leading to transcriptional activation. Ubiquitin as such does not act as an enzyme inhibitor through binding to active site but only through enzyme degradation. Regulation of Calcium pumps is through  Na-Ca exchangers and ATPases.