A carbon atom with four different substituents is said to be asymmetric, and asymmetric carbons are called chiral centers

Chiral carbon most distant from the carbonyl carbon has the same configuration as the chiral carbon in D-glyceraldehyde.

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A second class of CKIs found in metazoan cells is the Cip/Kip family of CKIs, consists of three proteins—p21CIP, p27KIP1, and p57KIP2. 

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The Cip/Kip family members bind to cyclin-bound CDKs and block the activity of cyclin D-, E-, A-, and B-containing complexes.

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Cyclins are degraded through the action of two different E3-ligases:

1. SCF (Skp1, Cullin, and F-box proteins) 

2. APC/C or cyclosome

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SCF and APC/C are multisubunit ubiquitin-protein ligases that belong to the RING finger family of ubiquitin-protein ligases.

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Human SCF complexes contain: 

Three core components - Cul1 subunit, RBX1, Skp1 

and a substrate-targeting F-box protein

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RBX1 is a RING finger–containing protein that binds the E2 ubiquitin-conjugating enzyme.