Distribution of amino acids in different proteins

[nomadreid]

TL;DR Summary

Do all human cells , or do all proteins, contain all the amino acids found in humans? For example, it appears that selenocysteine is in a very limited number of proteins?

In
https://en.wikipedia.org/wiki/Selenocysteine
, I read:
"As of 2021, 136 human proteins (in 37 families) are known to contain selenocysteine (selenoproteins)."
This seems to indicate (my knowledge of biochemistry being close to zero) that this amino acid is not found throughout the human body, but that the human body is programmed to take it up from the environment and use it for specific proteins. Is this way off base?

Are there other amino acids that are like this?

Thank you for your patience about the elementary nature of the question.

 

[pinball1970]

TL;DR Summary: Do all human cells , or do all proteins, contain all the amino acids found in humans? For example, it appears that selenocysteine is in a very limited number of proteins?

In
https://en.wikipedia.org/wiki/Selenocysteine
, I read:
"As of 2021, 136 human proteins (in 37 families) are known to contain selenocysteine (selenoproteins)."
This seems to indicate (my knowledge of biochemistry being close to zero) that this amino acid is not found throughout the human body, but that the human body is programmed to take it up from the environment and use it for specific proteins. Is this way off base?

Are there other amino acids that are like this?

Thank you for your patience about the elementary nature of the question.

Did you read this in your link? "Unlike the other amino acids, no free pool of selenocysteine exists in the cell. Its high reactivity would cause damage to cells.[19]"

I would also check this out. Which amino acids we get from diet,

https://en.wikipedia.org/wiki/Essential_amino_acid

Selenocysteine is a non essential amino acid. @jim mcnamara and Bill will correct/add hopefully.

 

[BillTre]

Proteins/amino acids:
Proteins are quite variable and molded by evolution for particular functions that are the result of the sequence of amino acids and how they fold into complex 3-D structures. The sequence is reliably folded in certain ways to produce a 3-D array of the chemical properties associated with each amino acid (aa) in the sequence as folded up. This creates a 3-D array of chemical properties at a molecular scale. Different between different kinds of proteins, but identical for proteins of the same kind.

Protein structure:
Different proteins will have different amino acid sequences (determined by their encoded sequence) generating different 3-D molecular structures with different chemical properties in different places. This lets different proteins have different properties to bind different molecules and catalyze different reactions.

Composition:
Different proteins can there for have different amino acid compositions for their different purposes. Some are made of only a few of the twenty different common amino acids.

Non-standard amino acids:
The selenocysteine is (from your wikipedia link) is not one of the standard amino acids. It is not coded for in sequence in the genome or the encoding mRNA. There is some interesting info on that in your link. The selenocysteine is inserted in the growing protein making unusual use of an otherwise stop codon with a non-standard tRNA and unusual secondary structure of the mRNA downstream of the stop codon.
Other non-standard amino acids can be modifications of normal proteins after the protein chain is made. The enzyme making the modifications would have to identify the the amino acid to modify out of all the others.

Metal uses:
Different metals provide additional chemical properties that amino acids alone can not provide. They can be important in catalyzing some reactions.

 

[nomadreid]

Thanks very much, pinball1970 and BillTre. Very interesting and informative, and an incentive to study these mechanisms further.