Biochemistry 2 Exam 4: Lecture 15 - Part 1

Insulin is classified as an _____.

Anabolic peptide hormone

How is insulin built?

Dimerized chains connected by disulfide bonds

Insulin is encoded by the ___ gene on chromosome __.

INS; 11

How many amino acids in the two insulin chains?

51 amino acids in the two chains that are connected by 2 disulfide bridges

Bovine insulin differs by ___ amino acids.

3

Porcine insulin differs by __ amino acids.

1

Insulin is synthesized as a precursor on the ____ of the pancreatic beta cell

Rough ER (pre-proinsulin)

Insulin is transported to the ____ and the signal sequence is ___, it folds and ____ are formed. 

ER lumen; cleaved; disulfide bonds (proinsulin)

Insulin leaves ER for the ____ and undergoes _____ cleavage of chain C to form a _____.

Golgi; proteolytic; dipeptide

Insulin and peptide C are complexed by ___ and packed in ____ in preparation for secretion.

Zinc ions; vesicles

The main stimulus of insulin secretion is ________ which is taken up by the _____ of pancreas via _____ transporters.

high blood glucose; beta cells; GLUT2

After insulin is taken up by the beta cells, it is ______ by ______, which acts as a _____.

Phosphorylated; glucokinase; glucose sensor

When glucose is metabolized it generates ___?

ATP

____ levels of ATP ____ potassium efflux through ATP dependent potassium channels in the cell. What is this caused by? 

High; block

Depolarization

T/F: In insulin release, depolarization opens calcium channels, increasing Ca2+ influx and increasing exocytosis of insulin.

True

Cellular response to insulin is mediated by insulin binding to ____ on target cell surface.

RTK (multiple pathways affected) 

What increases glucose uptake when the cell responds to insulin?

Akt signaling results in the translocation of GLUT4 to the cell membrane

How is glycogen formation increased as the cell responds to insulin?

PP-1 activation leads to increased glycogen synthase activity → increased glycogen formation

Cellular response to insulin has multiple effects on protein synthesis 

What does Ras/IRS-2 signaling pathway activation of transcription factors lead to?

Increased DNA synthesis, cell growth and division

Increased glycolytic, glucogenic and lipogenic enzymes.

What effects does insulin have on the liver?

Inhibits: glycogenolysis, conversion of FA/AA to keto acids, conversion of AA to glucose

Promotes: glucose storage as glycogen (induces glucokinase/glycogen synthase and inhibits glycogen phosphorylase)

Increases: triglyceride, cholesterol synthesis, and VLDL formation

Insulin effect on muscle

Increase: AA transport, protein synthesis, glucose transport, glycogen synthesis

Induces glycogen synthase and inhibits glycogen phosphorylase 

Effects of insulin on adipose tissue

Increase triglyceride storage

Actives lipoprotein lipase

Inhibits hormone sensitive lipase

How is glucagon classified?

Catabolic peptide hormone

T/F: Insulin is smaller than glucagon 

False - glucagon is smaller than insulin 

Glucagon is a _____ chain with no ______.

Single; disulfide bonds

What gene encodes for glucagon?

Gcg gene on chromosome 2

How many amino acids are in a single chain of glucagon?

29

Glucagon is synthesized as a _____ on the _____ of the pancreatic alpha cell.

Precursor; rough ER (pre-proglucagon)

Pre-proglucagon is transported to the ____ and the single sequence is cleaved to produce ______.

ER lumen; proglucagon

What else is proglucagon produced by?

Intestinal enteroendocrine cells and the brain

In the pancreas, prohormone convertase 2 (PC2) removes portions to make …

Glucagon

In the intestines and brain, prohormone convertase 1/3 (PC1/3) produces ______ peptide 1 and 2 (GLP-1 and GLP-2)

glucagon 

What is glucagon secretion triggered by?

Low blood glucose

In glucagon release, glucose is taken up by the ____ cell of pancreas via ____ transporter

alpha; GLUT1

In glucagon release, low levels of ___ block ____ efflux through ATP dependent _____ channels in the cell causing _____.

ATP; potassium; potassium; depolarization

During glucagon release, when the cell is depolarized what is the effect on calcium channels?

Channels open and increase influx of Ca2+ and increase exocytosis of glucagon

T/F: Insulin generates more ATP than glucagon.

True

What mediates the cellular response to glucagon?

G-protein linked receptors on target cell surface (glucagon binds to them)

What activates the pathway during the cellular response to glucagon?

PKA, which leads to phosphorylation of multiple enzymes

Effects of glucagon on liver

Simulates glycogenolysis and gluconeogenesis → induces glycogen phosphorylase and pyruvate carboxylase and inhibits glycogen synthase and pyruvate kinase/PFK-s

Inhibits: glycolysis, gluconeogenesis, FAS (ACC)

Stimulates FAO, ketone metabolism, and AA breakdown

Where does thyroid hormone synthesis take place?

Colloid of the thyroid

What does colloid contain?

Thyroglobulin (Tg)

Thyroglobulin is a large glycoprotein made up of:

Tyrosine residues, other AAs, and carbohydrates

The ____ gene is located on chromosome __ and is stimulated by TSH.

hTg; 8

Translation of the mRNA product occurs on the ___ of the follicular cells, glycosylation occurs in the golgi prior to exocytosis into the colloid

Rough ER

Where is iodide transported to?

Follicular cells via the sodium- iodide symporter (NIS)

T/F: when iodide is transported into the follicular cell, it is a secondary active transport and is coupled to the Na/K ATPase.

True

What does the transport of iodide allow for?

Maintenance of a high [I-] in thyroid vs serum

What is the transport of iodide into follicular cells stimulated by?

TSH and Grave’s antibodies (TSI)

How does iodide enter the colloid?

Passively via pendrin transport protein located on the apical cell surface

Oxidation of iodide to iodine intermediates via hydrogen peroxide is catalyzed by _______.

Thyroid peroxidase enzyme (TPO)

What is TPO?

Membrane bound glycoprotein with a prosthetic heme group

What is TPO production stimulated by?

TSH

Iodine intermediates are incorporated into ________ and form _____.

Thyroglobulin’s tyrosine residues; iodotyrosines

What are the two iodotyrosines?

Mono-iodotyrosine (MIT) and di-iodotyrosine (DIT)

Antibodies to TPO are pathognomonic for ______ 

Hashimoto’s thyroiditis 

In Hashimoto’s thyroiditis, antibodies against _____ can also occur

Thyroglobulin (ATA)

Iodotyrosine coupling:

DIT + DIT → Thyroxine (T4)

DIT + MIT → Triiodothyronine (T3) and some rT3

Thyroid hormone is secreted in response to _____ is _____ by follicular cells

TSH colloid; endocytosed 

In thyroid hormone secretion, colloid vesicles fuse with ____.

Lysosomes

Lysosomal proteolytic enzymes ____ the thyroglobulin contained within and releases __, __, __, and ___.

Hydrolyze; T3, T4, DIT and MIT

Where do T3, T4, and rT3 go after being relased?

Circulation 

What does an abundance of iodine or lithium carbonate interfere with?

Proteolysis

Wolff-Chaikoff Effect is …

A critical level of iodine supplementation is reached and hormone production is inhibited (Down-regulation of TPO mRNA)

T/F: In Wolff-Chaikoff effect, a normal healthy gland can escape hormone suppression via down-regulation of NIS mRNA

True

T/F: People with autoimmune etiology cannot make the adaptation in the Wolf-Chaikoff effect

True

Induction of hyperthyroidism after iodide load and is due to latent Grave’s disease.

Jod-Basedow Effect

Thyroid hormone is transported by _________ that bind the majority of hormones. 

Carrier proteins 

How much of T4 is thyroid bound? How much is unbound?

99.97%

0.03%

Only free T4 is active

How is T3 circulated?

In its bound form

T/F: Bound T3 is stronger than bound T4.

False, bound T3 is weaker than T4

T/F: T3 is more metabolically active than fT4

True, about 5x more active

What are the thyroid hormone carrier proteins?

Thyroid binding globulin (TBG)

Thyroxin-binding prealbumin

Albumin

How it most T3 produced?

Conversion of T4 in peripheral tissues via deiodinase enzymes 

What are the three types of deiodinase enzymes?

Types I, II, III

What are the characteristics of types I and II deiodinase?

They are 5’ deiodinases, they deiodinate the outer ring of T4, and increases activity

What are the characteristics of Type III deiodinase? 

5’ deiodinase, deiodinates the inner ring, and decreases activity. 

What do deiodinases do?

5’ deiodinases covert T4 to T3

What cofactor is required for deiodinase enzymes to work?

Selenium

Repletion of ____ Se increases enzyme activity, but if ______ is low, it can worsen the symptoms of ______.

Low; iodine; hypothyroidism 

What binds to intracellular receptors in the target tissue when thyroid hormone is released?

Triiodothyronine

When the cell responds to thyroid hormone, the hormone-receptor complex acts in the nucleus as a _______ and it affect the activity of a large number of genes.

Transcription factor

What effect does thyroid hormone have on its target tissue?

Elevates cell/tissue basal metabolic rates

Promotes: carbohydrates over fat for fuel, AA uptake and protein synthesis during cell growth

Affects synthesis, degradation, and intermediate metabolism of lipids