Cell Cycle
Post-translational modifications (PTMs) are covalent modifications of amino acid side chains
(N/C-terminus) which increase diversity of proteins beyond the 20 amino acids. They can serve
as a reversible switch modifying the enzymes, or de-modifying them.
Protein Phosphorylation - the transfer of y-phosphoryl group (phosphorus plus 3 oxygens) from
ATP to serine, threonine, or tyrosine residues. This produces a phosphorylated animo acid.
Nearly all cellular processes are regulated by phosphorylation.
Protein kinases transfer a phosphoryl group from ATP to a target protein. There are two classes:
Ser/Thr Kinases
Tyr Kinases
Both ATP and the target protein are both substrates as protein kinase binds to both. It then
releases a phosphate group phosphorylating the protein.
Most protein kinases have a similar structure and they are regulated by accessory proteins.
The substrate proteins have a kinase-specific recognition sequence adjacent to phosphorite
(consensus sequence). The term consensus sequence refers to those sequence elements
immediately surrounding the sites phosphorylated by a given protein kinase that are considered
essential for its recognition and phosphorylation by that kinase. The substrate proteins may also
bind kinase independent of consensus.
The addition of the phosphoryl group changes the chemistry (microenvironment) of the modified
residue. It can change:
Protein conformation (secondary/tertiary structure) and surface charge or shape.
The changes can promote or impair enzyme activity and protein-protein interactions.
Cyclin-dependent kinase (CDK) regulates the cell cycle. It works in chromosome condensation,
nuclear envelope disassembly, mitotic spindle assembly and function, and cytokinesis. The CDK
is only active when bound to a cyclin regulator protein. The proteins it works on are:
Chromosomal Proteins
Nuclear Lamins
Microtubule-associated proteins
Cell Cycle 1
, Other protein kinases
Many substrates contain the consensus sequence S/T-P-X-K/R.
Beginning Mitosis
Cyclins are proteins which control the cell cycle regulating the activity of CDK’s.
CDK’s are cyclic dependent kinases which regulate the cell cycle as an accumulation of CDK
causes mitosis to begin.
CDK is regulated by phosphorylation where phosphate group inhibits the cyclin-CDK complex.
The phosphatase enzyme Cdc25 removes the inhibitory phosphorylation activating the CDK.
This allows mitosis to occur.
The process involves positive feedback where the CDK-cyclin complex causes more Cdc25 to
dephosphorylate the complex.
Leaving Mitosis
Decreased CDK activity due to decreased cyclin levels. The cyclin protein is then degraded
through proteolysis (breakdown of proteins into smaller polypeptides). Another PTM which
causes ubiquitylation (protein ubiquitin is added to a protein) leads to cyclin proteolysis.
1. Active CDK and cyclin form a complex.
2. Chromosome alignment in metaphase spindle leads to activation of the multiprotein
ubiquitin ligase; Anaphase Promoting Complex (APC).
3. APC polyubiquitylates cyclin.
4. Polyubiquitated cyclin is recognised by proteasome and degraded into small peptides.
5. Inactive CDK remains.
Regulated Proteolysis: Sister Chromatid Separation
Sister chromatids are linked by cohesin rings which hold linking DNA together in radial loops.
The enzyme separase allows the sister chromatids to separate.
1. Separase is prevented by the inhibitory protein securin which blocks the separase from being
active.
2. When the APC is activated (and cyclin is activated), it ubiquitylates the securin protein
allowing separase to be activated.
Cell Cycle 2
Post-translational modifications (PTMs) are covalent modifications of amino acid side chains
(N/C-terminus) which increase diversity of proteins beyond the 20 amino acids. They can serve
as a reversible switch modifying the enzymes, or de-modifying them.
Protein Phosphorylation - the transfer of y-phosphoryl group (phosphorus plus 3 oxygens) from
ATP to serine, threonine, or tyrosine residues. This produces a phosphorylated animo acid.
Nearly all cellular processes are regulated by phosphorylation.
Protein kinases transfer a phosphoryl group from ATP to a target protein. There are two classes:
Ser/Thr Kinases
Tyr Kinases
Both ATP and the target protein are both substrates as protein kinase binds to both. It then
releases a phosphate group phosphorylating the protein.
Most protein kinases have a similar structure and they are regulated by accessory proteins.
The substrate proteins have a kinase-specific recognition sequence adjacent to phosphorite
(consensus sequence). The term consensus sequence refers to those sequence elements
immediately surrounding the sites phosphorylated by a given protein kinase that are considered
essential for its recognition and phosphorylation by that kinase. The substrate proteins may also
bind kinase independent of consensus.
The addition of the phosphoryl group changes the chemistry (microenvironment) of the modified
residue. It can change:
Protein conformation (secondary/tertiary structure) and surface charge or shape.
The changes can promote or impair enzyme activity and protein-protein interactions.
Cyclin-dependent kinase (CDK) regulates the cell cycle. It works in chromosome condensation,
nuclear envelope disassembly, mitotic spindle assembly and function, and cytokinesis. The CDK
is only active when bound to a cyclin regulator protein. The proteins it works on are:
Chromosomal Proteins
Nuclear Lamins
Microtubule-associated proteins
Cell Cycle 1
, Other protein kinases
Many substrates contain the consensus sequence S/T-P-X-K/R.
Beginning Mitosis
Cyclins are proteins which control the cell cycle regulating the activity of CDK’s.
CDK’s are cyclic dependent kinases which regulate the cell cycle as an accumulation of CDK
causes mitosis to begin.
CDK is regulated by phosphorylation where phosphate group inhibits the cyclin-CDK complex.
The phosphatase enzyme Cdc25 removes the inhibitory phosphorylation activating the CDK.
This allows mitosis to occur.
The process involves positive feedback where the CDK-cyclin complex causes more Cdc25 to
dephosphorylate the complex.
Leaving Mitosis
Decreased CDK activity due to decreased cyclin levels. The cyclin protein is then degraded
through proteolysis (breakdown of proteins into smaller polypeptides). Another PTM which
causes ubiquitylation (protein ubiquitin is added to a protein) leads to cyclin proteolysis.
1. Active CDK and cyclin form a complex.
2. Chromosome alignment in metaphase spindle leads to activation of the multiprotein
ubiquitin ligase; Anaphase Promoting Complex (APC).
3. APC polyubiquitylates cyclin.
4. Polyubiquitated cyclin is recognised by proteasome and degraded into small peptides.
5. Inactive CDK remains.
Regulated Proteolysis: Sister Chromatid Separation
Sister chromatids are linked by cohesin rings which hold linking DNA together in radial loops.
The enzyme separase allows the sister chromatids to separate.
1. Separase is prevented by the inhibitory protein securin which blocks the separase from being
active.
2. When the APC is activated (and cyclin is activated), it ubiquitylates the securin protein
allowing separase to be activated.
Cell Cycle 2
