EXAM #1: NORMAL BIOCHEMISTRY AND PHYSIOLOGY

Question 1

What is the precursor of the thyroid hormones?

Answer 1

Tyrosine

Question 2

What is the active form of thyroid hormone? Which is more active?

Answer 2

T3* and T4

*T3 is more active

Question 3

What are the biologicaly inactive forms of the thyroid hormones?

Answer 3

T2 and rT3

Question 4

Where are thyroid hormones synthesized?

Answer 4

Thyroid follicle

Question 5

What direction does the apical side of the follicular cell face? Basal?

Answer 5

Apical= colloid

Basal= bloodstream

Question 6

What is the platform for T3 and T4 synthesis?

Answer 6

Thryoglobulin

Question 7

What is exogenous substance is required for the synthesis of thyroid hormones?

Answer 7

Iodine

Question 8

What has virtually eliminated iodine deficiency in the US?

Answer 8

Iodized salt

Question 9

How is iodine transported into the thyroid follicle?

Answer 9

Na+-I cotransporter (symporter)

Question 10

What is organification?

Answer 10

Adding iodine to thyroglobulin

Question 11

What is the site of organification?

Answer 11

Exocytotic vesicles in the colloid

Question 12

Outline the process of organification.

Answer 12

Thyroid peroxidase oxidizes and attaches iodide to the tyrosyl residues

*Uses hydrogen peroxide

Question 13

What is the outcome of organification?

Answer 13

Formation of MIT and DIT

*MIT= 1 iodine 
DIT= 2x iodine

Question 14

What is the next step after organification?

Answer 14

Coupling

Question 15

What enzymes is responsible for coupling?

Answer 15

Thyroid peroxidase (TPO)

Question 16

How is thyroglobulin released via the lysosomal pathway?

Answer 16

1) Endocytosis
2) Hydrolysis in the lysosome/ proteolysis
3) Diffusion into the circulation

Question 17

What happens to thyroglobulin after cleavage of T3 and T4 during release?

Answer 17

1) Recycled to colloid

2) Transcytosis into the circulation

Question 18

Describe the composition of TRH.

Answer 18

Pyroglutamul-histidyl-proline residues

Question 19

What is the mechanism of TRH signaling in the anterior pituitary?

Answer 19

GPRC via

• PLC
• IP3
• Ca++

Question 20

What is the mechanism of TSH signaling in the thyroid gland?

Answer 20

GPRC via

- cAMP protein kinases

Question 21

Specifically, what does TSH stimulate?

Answer 21

1) Iodide transport
2) Transcription of thyroglobulin
3) Transcruption of tyroid peroxidase (TPO)

Question 22

What thyroid hormone is preferentially bound to serum proteins?

Answer 22

T4

Question 23

What enzymes catalyze the metabolism of T3 and T4?

Answer 23

Deiodinases

Question 24

What are the functions of deiodinases?

Answer 24

1) Formation of active T3 from T4

2) Deactivation of T3 and T4 when their concentrations are too high

Question 25

What form of deiodinase is active against T3?

Answer 25

Type III *Thus, this is the major enzyme that deactivates thyroid hormones

Question 26

What is the function of Type I deiodinase?

Answer 26

Outer/ inner ring vs. T4

Question 27

Where is Type I deiodinase located?

Answer 27

Liver, kidney, thyroid

Question 28

What are the products of Type I deiodinase?

Answer 28

T3 or rT3

Question 29

What is the function of Type II deiodinase?

Answer 29

Outer ring vs. T4

Question 30

What is the location of Type II deiodinase?

Answer 30

Diffuse

Question 31

What is the product of Type II deiodinase?

Answer 31

Intracellular T3

Question 32

What is the function of Type III deiodinase?

Answer 32

Inner ring vs. T4 and T3

Question 33

Where is Type III deiodinase located?

Answer 33

Brain and placenta

Question 34

What are the products of Type III deiodinase?

Answer 34

rT3 and T2

Question 35

Describe the mechanism of action of the thyroid hormones in the nucleus.

Answer 35

1) TR binds DNA with RXR 2) TR/RXR recruits HDAC, repressing transcription 3) Binding of T3 or T4 displaces HDAC for HAT* HAT complex turns target genes ON

Question 36

What is stimulated by T3/T4 in regards to cellular metabolism?

Answer 36

1) Increased oxygen consumption/ heat production 2) Increased adipose tissue lipolysis 3) Increased glycogenolysis and gluconeogenesis