Peptide Hormones

[MedPR]

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EK: "Since they are peptide derivatives, peptide hormones are water soluble.."

Are they water soluble because of the carboxylic acid group and amino group? Is there anything else that contributes to it?

 

[Ssina]

They are mini-proteins, so aa with polar side groups sticking out

 

[Erythropoietin]

COOH and RNH2 are the major contributors to water solubility.
I believe some of them (like oxytocin) have disulfide bridges, so that might be another reason.

 

[MT Headed]

Any peptide backbone has a lot of C=O carbonyls and N-H groups greatly adding to water solubility, even if the side chains were hydrophobic like a bunch of valines.

 

[MedPR]

Any peptide backbone has a lot of C=O carbonyls and N-H groups greatly adding to water solubility, even if the side chains were hydrophobic like a bunch of valines.

What about a steroid with 4 carbonyls on it? I don't remember where, but I did a discrete with an image of a structure that looked like cholesterol, but it had 2 cyclohexanes and 1 cyclopentane. I initially thought it would be lipid soluble, since it was very similar to cholesterol and probably a steroid, but I second guessed it because it had multiple COOH groups. The answer said it was lipid soluble.

 

[MT Headed]

Sounds like a mean question to me. Hopefully the real mcat will be more obvious. At some point if you keep adding carboxyl groups to a steroiddy molecule it will be water soluble. Sounds like you know what's going on here, that's the important thing.

 

[dranet]

The major reason for water solubility is the ability to form Hydrogen-bonds. So if you have F, O, N you can H-bond. Peptide hormones have lots of Oxygen and Nitrogen to H-bond too. The lipid soluble hormones do not but if any have such a characteristic Med PR described you will find an ability to H-bond, another MCAT curveball. Hope that helps.

 

[MedPR]

Sounds like a mean question to me. Hopefully the real mcat will be more obvious. At some point if you keep adding carboxyl groups to a steroiddy molecule it will be water soluble. Sounds like you know what's going on here, that's the important thing.

Thanks! The encouragement (whether you realize it or not) coming from you is very.. encouraging. You seem to have the answer for every question in every subject.

The major reason for water solubility is the ability to form Hydrogen-bonds. So if you have F, O, N you can H-bond. Peptide hormones have lots of Oxygen and Nitrogen to H-bond too. The lipid soluble hormones do not but if any have such a characteristic Med PR described you will find an ability to H-bond, another MCAT curveball. Hope that helps.

👍

 

[Erythropoietin]

What about a steroid with 4 carbonyls on it? I don't remember where, but I did a discrete with an image of a structure that looked like cholesterol, but it had 2 cyclohexanes and 1 cyclopentane. I initially thought it would be lipid soluble, since it was very similar to cholesterol and probably a steroid, but I second guessed it because it had multiple COOH groups. The answer said it was lipid soluble.

You have to think conceptually. If you have a structure that contains 10 fused rings, then it is entirely nonpolar. If you have another structure that has 10 fused rings plus one OH functional group, then it is slightly water soluble. If you have another structure that has 10 fused rings plus two OH groups, then it is more water soluble.
I am sure that question did not contain 4 different cholesterol structures in the answers.

 

[MedPR]

You have to think conceptually. If you have a structure that contains 10 fused rings, then it is entirely nonpolar. If you have another structure that has 10 fused rings plus one OH functional group, then it is slightly water soluble. If you have another structure that has 10 fused rings plus two OH groups, then it is more water soluble.
I am sure that question did not contain 4 different cholesterol structures in the answers.

The question had an image of a "hormone" with 2 cyclohexanes and 1 cyclopentane with 4 carbonyls and asked something about how it would act on the effector.

I was torn between secondary messenger (for water soluble hormones) or binding to a receptor in the cytosol (for steroids/lipid soluble hormones).

 

[GoZags]

The question had an image of a "hormone" with 2 cyclohexanes and 1 cyclopentane with 4 carbonyls and asked something about how it would act on the effector.

I was torn between secondary messenger (for water soluble hormones) or binding to a receptor in the cytosol (for steroids/lipid soluble hormones).

I would tend to disagree with solubility here for small peptides, I think it depends more on the amino acids contained within, lots of greasy/bulky side chains = very low solubility. Ever try to dissolve, say, phenylalanine in water? Its solubility stinks. Yes only one AA by itself, but nonetheless sort of a litmus test that can sometimes be extrapolated to a larger peptide with lots of greasy side chains.

About your hormone you're describing, if it has 2 cyclohexanes, 1 cyclopentane, I would tend to think it can bypass the cellular membrane due to its planar structure (much like a steroid). Some food for thought, perhaps I've missed the mark.