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Lysine biochemistry & reactivity - emphasis on its charge & nucleophilic ability to form bonds. Move over Kelloggs, this “K” is way more special than your cereal! Why? For one, the amino acid (protein letter) Lysine (Lys, K) seems to have a random abbreviation… but that’s just cuz the more “logical” ones were taken, & K was closest to L in alphabet! That’s not what makes lysine really special… It can exist in positive-charged or neutral states - cool but not unique (arginine and histidine have that superpower too). Nope - what really makes lysine special is its ability to form “Schiff bases” that can provide reversible yet strong linkages between proteins and other things like helper molecules (cofactors) that help it speed up (catalyze) many a biochemical reaction. And amide bonds to other things like modifying groups like ubiquitin (which can tag proteins for degradation) or methyl or acetyl groups (which can alter gene expression). So let’s take a look at lysine in action!
blog form: bit.ly/lysinean... ; KZbin: • Lysine let’s you link ...
It’s Day 12 of #20DaysOfAminoAcids - the bumbling biochemist’s version of an advent calendar. Amino acids are the building blocks of proteins. There are 20 (common) genetically-specified ones, each with a generic backbone with to allow for linking up through peptide bonds to form chains (polypeptides) that fold up into functional proteins, as well as unique side chains (aka “R groups” that stick off like charms from a charm bracelet). Each day I’m going to bring you the story of one of these “charms” - what we know about it and how we know about it, where it comes from, where it goes, and outstanding questions nobody knows.
More on amino acids in general here bit.ly/aminoaci... but the basic overview is:
amino acids have generic “amino” (NH₃⁺/NH₂) & “carboxyl” (COOH/COO⁻) groups that let them link up together through peptide bonds (N links to C, H₂O lost, and the remaining “residual” parts are called residues). The reason for the “2 options” in parentheses is that these groups’ protonation state (how many protons (H⁺ ) they have) depends on the pH (which is a measure of how many free H⁺ are around to take).
Those generic parts are attached to a central “alpha carbon” (Ca), which is also attached to one of 20 unique side chains (“R groups”) which have different properties (big, small, hydrophilic (water-loving), hydrophobic (water-avoided), etc.) & proteins have different combos of them, so the proteins have different properties. And we can get a better appreciation and understanding of proteins if we look at those letters. So, today let’s look at Lysine!
But before we get into examining the special part, let’s look a little closer at the generic part. All amino acids have that nitrogen (N) attached to the central, alpha carbon (Ca) and its this nitrogen that gives them their “alpha-amino” designation. “Amine” is a name we give to nitrogen attached to things and we can further classify amines as primary (attached through a single bond to a single carbon-containing group (abbreviated C)), secondary (2 links to carbon), tertiary (3 links), or quaternary (4 links to C).
In their free state, all amino acids (except for proline because its side chain wraps around and binds its N) are “primary amines” - they have primary α-amino groups. When amino acids join up through peptide bonds, that N gets attached to a C from its neighbor, so it becomes a secondary amine.
Lysine has an additional primary amino group at the end of its side chain. Speaking of that side chain, Lys’s side chain is a “butylamine.” The “butyl” refers to the 4 methylene (-CH₂) linker between the Ca it sticks off from and that end amino group. The atoms of K’s side chain are sometimes given numbers starting w/the carboxylate C of the backbone (in this method, the amino group is attached to C₆) & sometimes, instead of numbers, they’re given Greek letters starting with the backbone’s C (in this method, the amino group is attached to C𝜀). Because of this Greek lettering scheme, lysine is often described as having an ε-amino group.
Under physiological (normal bodily) conditions, the nitrogen of this amino group is attached to as many hydrogen atoms as possible (i.e. it is fully protonated). For a primary amino group, this means being bound to 3 H’s, and it leads to a positive charge (-NH₃⁺). Why?
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