Please I need help with biochemistry

Question

selenocysteine and pyrrolysine. Based on their structure, what unique reactions/weak interactions can they engage in? Why are they called the 21th and 22th amino acids

Answer 1

Part 1

cysteine has sulfur

Selenocysteine has selenium. Selenium holds its electrons more loosely than sulfur does making selenium better than cysteine at giving, taking, and sharing electrons.

***What unique reactions/weak interactions can they engage in?***

Selenocysteine (Sec)

1. better nucleophile (attacks other molecules more easily) than cysteine
2. faster reactions for redox than cysteine
3. better electron transfer than cystein

lysine has a long chain

Pyrrolysine (Pyl) has a ring added. That ring gives it a reactive spot that can temporarily bond to other molecules.

***What unique reactions/weak interactions can they engage in?***

Pyrrolysine (Pyl)

1. Form Schiff bases (temporary covalent bonds enzymes use to hold onto substrates)
2. stabilize intermediates (hold molecules together during reaction)
3. help with methyl-transfer reactions (make/break)

Part 2

There are 2 types of amino acids in proteins:

1. amino acids are added to the protein as it's being made on the ribosome
2. amino acids are added to the protein AFTER it's been translated by chemical modification

Standard 20 amino acids are defined by:

1. Have their own codons in mRNA

2. Have their own tRNAs

3. Are inserted directly by the ribosome during translation

4. Are used universally by almost all organisms

These amino acids are added to the protein as it's being made on the ribosome.

***Why are they called the 21st and 22nd amino acids?***

These two amino acids are added to the protein as it's being made on the ribosome.

They have:

1. Have their own codons in mRNA
2. Have their own tRNAs
3. Are inserted directly by the ribosome during translation
4. Sec is commonly found in many organisms but not all. Pyl is found in less organisms.