67 - Steroid Hormones

Question 1

Ring structure of steroid hormones

Answer 1

Three hexagonal rings, one pentagonal ring.

Question 2

Pathway from cholesterol to progesterone

Answer 2

Cholesterol -> Pregnenolone -> Progesterone

Question 3

Cortisol synthesis

Answer 3

Cholesterol -> Pregnenolone -> Progesterone -> Cortisol

Question 4

Estradiol synthesis

Answer 4

Cholesterol -> Pregnenolone -> Progesterone -> Testosterone -> Estradiol

Question 5

Pregnenolone

Answer 5

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.

Question 6

Conversion of pregnenolone to progesterone

Answer 6

1. The 3-hydroxyl group is oxidised to a 3-keto group

2. The Δ5 double bond is isomerised to Δ4 double bond

Question 7

Regulation of synthesis and secretion of glucocorticoids

Answer 7

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.

Question 8

Histone structure
1
2
3

Answer 8

• 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

Question 9

Function of acetylation of histones

Answer 9

Positive charge of parts of histones mediate binding to DNA.

Acetylation negates this charge, reducing linkage of DNA and histone.

Question 10

ATP hydrolysis mechanism of histone-DNA unwinding

Answer 10

Nucleosome remodelling ATPase.

Binds to core nucleosome and DNA, hydrolyses ATP.

Question 11

Enzyme responsible for phosphorylation of histones

Answer 11

Cyclin-dependent kinase

Question 12

Enzyme responsible for acetylation of histones

Answer 12

Histone acetyltransferase

Question 13

Examples of co-repressor molecules
1
2

Answer 13

H1 phosphatase

Histone deacetylases

Question 14

Two major groups of transcription factors

Answer 14

1. Non-gene specific transcription factors (TFII family) associated with RNA polymerase II complex
2. Gene-specific transcription factors

Question 15

Example of gene-specific transcription factors (nuclear)
1
2
3
4

Answer 15

• 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

Question 16

Example of a non-gene specific transcription factor

Answer 16

RNA polymerase II (will transcribe any open area of DNA).

Question 17

Major function of RNA polymerase II

Answer 17

Assemble transcription initiation complex at the transcriptionstart sites

Question 18

Domain structure of a typical transcription factor
1
2
3

Answer 18

Regulatory domain (ligand-binding domain)
DNA binding domain
Transactivator domain

Question 19

What do gene-specific transcription factors bind to?

Answer 19

Response elements within specific genes

Question 20

Two distinct classes of nuclear receptors

Answer 20

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.

Question 21

Manner in which class I nuclear receptors interact with DNA

Answer 21

Bind two antiparallel DNA sequences (EG: AGGTCA …. TGACCT)

Question 22

Manner in which class II nuclear receptors interact with DNA

Answer 22

Bind two identical DNA sequences that are in sequence with each other (EG: AGGTCA … AGGTCA)

Question 23

Arrangement of zinc fingers in class I nuclear receptors

Answer 23

Mirror images of each other

Question 24

Arrangement of zinc fingers in class II nuclear receptors

Answer 24

Repeats of one another

Question 25

Structure in nuclear receptors that bind to DNA

Answer 25

Zing fingers (four cysteine residues co-ordinating a zinc)

Question 26

Functions of chaperones with nuclear receptor class I

Answer 26

• Assist folding of nascent receptor
• Prevents degradation
• Maintain ligand-binding ability

Question 27

Activation of gene via class I nuclear receptor
1
2
3

Answer 27

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

Question 28

Activation of gene via class II nuclear receptor

Answer 28

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.

Question 29

Activation function 1 and 2 sites of nuclear hormones

Answer 29

1 - Binding to co-activators and phosphorylation
2 - Ligand binding Binding to consensus
LxxLL motif in co-activators
Dimerisation

Question 30

Steroid receptor co-activator (SRC) family structure
1
2
3

Answer 30

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

Question 31

Examples of co-activators that can bind to SRC

Answer 31

Acetyltransferases
Ubiquitin ligases
ATPases
Methylases
Cell cycle regulators

Question 32

Example of co-repressors that can bind RxR/TR dimer 
1
2
3
4

Answer 32

• 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.

Question 33

Non-genomic action of progesterone

Answer 33

Has a receptor on the plasma membrane (GPCR).

Leads to breakdown of nuclear membrane in xenopus oocytes.