Thyroxine and T3

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So the logic below is apparently where my research has taken me. Someone please point out which reasoning step is false or reconcile the paradox.

1. Peptide hormones are water-soluble and diffuse through blood.
2. T3 and T4 are peptide hormones (synthesized from the amino acid tyrosine).
3. But, T3 and T4 require transport proteins to dissolve through blood.


My guess is that #1 is wrong. In other words, peptide hormones (like amino acids) are not necessarily polar. I'm just confused because I have heard #1 from numerous sources (including a Kaplan teacher).

Anyways, thanks in advance

 

[michaelfranklin]

While, Choice 1 may sounds like a bit of a generalization, but from what I understand, when dealing with hormones you classify them as being either water soluble or lipid soluble.

All the peptide hormones that we are expected to know for the MCAT are for the most part considered to be water soluble. The important thing is that they usually bind to extracellular receptors. The tyrosine derived hormones, although derived from an amino acid, are in a class of their own (just like amino acid transmitters and norepinephrine are group differently as NT's)

Lipid soluble hormones are steroid derived hormones (Aldosterone, Cortisone, Testosterone, Estrogen) and T3/T4. These bind to intracellular receptors.

 

[flodhi1]

So the logic below is apparently where my research has taken me. Someone please point out which reasoning step is false or reconcile the paradox.

1. Peptide hormones are water-soluble and diffuse through blood.
2. T3 and T4 are peptide hormones (synthesized from the amino acid tyrosine).
3. But, T3 and T4 require transport proteins to dissolve through blood.


My guess is that #1 is wrong. In other words, peptide hormones (like amino acids) are not necessarily polar. I'm just confused because I have heard #1 from numerous sources (including a Kaplan teacher).

Anyways, thanks in advance

I hope I address your question from what I am understanding is that you're confused how peptide hormones are water soluble and how T3 and T4 are NOT water soluble.
Now I want to start off by saying I don't like the fact that Kaplan identifies them as "amino acid derivatives" it really confuses people but unfortunately thats the name. I like to classify them as Tyrosine derivatives (Since Tyrosine is the Amino acid that derives both Catecholamines (Epenephrine) and (T3, T4). The thing is that TYROSINE derivatives can either be soluble like Epenephrine or be insoluble and require a carrier which are the thyroid hormones.

So we have 1) Peptide hormones - water soluble bind to receptors on the cell membrane
2) Steroid hormones bind to receptors in the cytoplasm or nucleus
3) Tyrosine derivatives with epinephrine binding to rceptors on the membrane like peptide hormones but Thyroid hormones binding to receptors in the nucleus.

For even greater detail
Peptide hormones:-
Lipophobic (thus unable to penetrate through cell membrane)
Previously produced + stored in vesicles
Released by exocytosis
Water soluble – easily transported in blood
Short half-life
Membrane receptor
2nd messenger system (cAMP very common)
others include: activate tyrosine kinase
turn on/off gene expression
open ion channels
Rapid cellular response – 2nd messenger modifies existing proteins

In conclusion all statements are CORRECT you're just confusing the TYROSINE amino acid derivative T3 and T4 with Peptide hormones. Those as two separate things.