67 - Steroid Hormones Flashcards
Ring structure of steroid hormones
Three hexagonal rings, one pentagonal ring.
Pathway from cholesterol to progesterone
Cholesterol -> Pregnenolone -> Progesterone
Cortisol synthesis
Cholesterol -> Pregnenolone -> Progesterone -> Cortisol
Estradiol synthesis
Cholesterol -> Pregnenolone -> Progesterone -> Testosterone -> Estradiol
Pregnenolone
A precursor of steroid hormones, is formed by cleavage of the side chain of cholesterol.
- Conversion of the 27-carbon cholesterol to the 21-carbon pregnenolone
- Hydroxylation reaction: NADPH and O2 are involved, requires cytochrome P450
- Step 1: removal of a 6-carbon unit from the side chain
- The step is activated by ACTH and Angiotensin II.
Conversion of pregnenolone to progesterone
- The 3-hydroxyl group is oxidised to a 3-keto group
2. The Δ5 double bond is isomerised to Δ4 double bond
Regulation of synthesis and secretion of glucocorticoids
Circadian rhythm and stressors stimulate PVN to release CRH.
Adrenocorticotrophic hormone released from anterior pituitary.
Stimulates adrenal cortex to release corticosterone.
Corticosterone negatively feeds back on hypothalamus and anterior pituitary.
Histone structure
1
2
3
- All histones contain a conserved C-terminal region mediating histone-histone interactions and histone-DNA interactions
- In chromatin, DNA wrapped around the core histone to formnucleosomes with 146 bp and 2xH2a, H2b, H3 & H4.
- H1 (linking nucleosome) binds to linker DNA to further impose compaction of the neigbouring nucleosomes
Function of acetylation of histones
Positive charge of parts of histones mediate binding to DNA.
Acetylation negates this charge, reducing linkage of DNA and histone.
ATP hydrolysis mechanism of histone-DNA unwinding
Nucleosome remodelling ATPase.
Binds to core nucleosome and DNA, hydrolyses ATP.
Enzyme responsible for phosphorylation of histones
Cyclin-dependent kinase
Enzyme responsible for acetylation of histones
Histone acetyltransferase
Examples of co-repressor molecules
1
2
H1 phosphatase
Histone deacetylases
Two major groups of transcription factors
- Non-gene specific transcription factors (TFII family) associated with RNA polymerase II complex
- Gene-specific transcription factors
Example of gene-specific transcription factors (nuclear) 1 2 3 4
- Nuclear receptors (e.g. Steroid hormone and TH receptors) are ligand-activated transcription factors are the target of gene-specific regulatory signals
- Interact with specific regulatory gene sequences (Response Elements)
- Organise the assembly of RNA polymerase II complex and co-activators to initiate transcription of specific genes.
- Organise the assembly of co-repressors to suppress transcription of specificgenes
Example of a non-gene specific transcription factor
RNA polymerase II (will transcribe any open area of DNA).
Major function of RNA polymerase II
Assemble transcription initiation complex at the transcriptionstart sites
Domain structure of a typical transcription factor
1
2
3
Regulatory domain (ligand-binding domain) DNA binding domain Transactivator domain
What do gene-specific transcription factors bind to?
Response elements within specific genes
Two distinct classes of nuclear receptors
Class I - Mostly steroid hormone receptors, form homodimers, unliganded form is in cytosol, complexed with chaperones.
Class II - Mostly thyroid hormone receptors, heterodimerise with RXR, unliganded form bound to target genes in nucleus and bound to co-repressors.
Manner in which class I nuclear receptors interact with DNA
Bind two antiparallel DNA sequences (EG: AGGTCA …. TGACCT)
Manner in which class II nuclear receptors interact with DNA
Bind two identical DNA sequences that are in sequence with each other (EG: AGGTCA … AGGTCA)
Arrangement of zinc fingers in class I nuclear receptors
Mirror images of each other
Arrangement of zinc fingers in class II nuclear receptors
Repeats of one another
Structure in nuclear receptors that bind to DNA
Zing fingers (four cysteine residues co-ordinating a zinc)
Functions of chaperones with nuclear receptor class I
- Assist folding of nascent receptor
- Prevents degradation
- Maintain ligand-binding ability
Activation of gene via class I nuclear receptor
1
2
3
1) Inactive receptor with chaperone complex is activated by steroid hormone
2) Class I receptor dimerises, enters nucleus
3) Homodimer binds co-activator, mRNA transcribed
Activation of gene via class II nuclear receptor
Bound to response element of target gene.
Without ligand (EG: T3), co-ordinates a co-repressor complex.
Upon binding to ligand, co-ordination of a co-activator complex.
Activation function 1 and 2 sites of nuclear hormones
1 - Binding to co-activators and phosphorylation
2 - Ligand binding Binding to consensus
LxxLL motif in co-activators
Dimerisation
Steroid receptor co-activator (SRC) family structure
1
2
3
1) Basic helix-loop-helix motif allows binding to DNA
2) LxxL motifs for nuclear receptor binding
3) PAS domain for dimerisation and binding to other nuclear proteins
Examples of co-activators that can bind to SRC
Acetyltransferases Ubiquitin ligases ATPases Methylases Cell cycle regulators
Example of co-repressors that can bind RxR/TR dimer 1 2 3 4
- Silencing mediators of thyroid receptor (SMRT)
- Nuclear Receptor Co-repressor (NCoR)
- Binding to nuclear receptors depends on their AF2 conformation
- Recruits histone deacetylases (HDACs) to silence the target genes,keeping them in the inactive compact nucleosome state.
Non-genomic action of progesterone
Has a receptor on the plasma membrane (GPCR).
Leads to breakdown of nuclear membrane in xenopus oocytes.
