Hydrogen Donor
Nattokinase

In the serine protease subtilisin, the amino acids aspartic acid, histidine, and serine form a catalytic triad. Serine acts as the primary hydrogen donor to the histidine residue, which in turn is stabilized by the aspartic acid residue.

Roles of the Catalytic Triad in Subtilisin:

The three residues work in concert through a charge-transfer system to facilitate the hydrolysis of peptide bonds.

Serine (Ser221 in subtilisin):

The hydroxyl group of serine is the nucleophile that directly attacks the substrate's peptide bond. It acts as a hydrogen donor to the histidine in the initial steps of the reaction mechanism, becoming a highly reactive alkoxide ion.

Histidine (His64 in subtilisin):

This residue acts as a general acid/base. It accepts a proton from the serine, thereby activating it for nucleophilic attack. It later donates this proton to the leaving group (the N-terminus of the cleaved peptide) and then abstracts a proton from a water molecule during the deacylation phase.

Aspartic Acid (Asp32 in subtilisin):

This residue's primary role is to orient and stabilize the histidine residue through hydrogen bonding. The negative charge of the aspartate carboxylate group helps to stabilize the positive charge that forms on the histidine during the catalytic cycle, which enhances the histidine's ability to abstract a proton from serine and makes the serine a better nucleophile.

The arrangement of these three residues, despite having different positions in the primary sequence compared to other serine proteases like chymotrypsin, creates an identical 3D active site arrangement, a classic example of convergent evolution.

..

In the serine protease subtilisin, the aspartic acid, histidine, and serine residues form a catalytic triad that works together to perform hydrolysis of peptide bonds. The triad functions as a charge-relay system to make the serine a potent nucleophile.

Roles of the Catalytic Triad in Subtilisin
Serine:

This residue acts as the primary nucleophile. Its hydroxyl group is activated to perform a nucleophilic attack on the carbonyl carbon of the substrate's peptide bond, which is the key step in breaking the bond.

Histidine:

The histidine residue acts as a general base and then a general acid during the reaction cycle. It accepts a proton from the serine's hydroxyl group, increasing the serine's reactivity. Later in the reaction (during deacylation), it acts as a hydrogen donor to the leaving group and then a base again to activate a water molecule for the second nucleophilic attack.

Aspartic Acid:

The aspartic acid's carboxyl group helps to correctly orient the histidine residue and stabilize the positive charge that develops on the histidine during the reaction's transition states. It makes the histidine a more effective base and prevents the accumulation of an unstable positive charge, which is crucial for efficient catalysis.

This spatial arrangement, along with an "oxyanion hole" that stabilizes the negatively charged tetrahedral intermediate that forms during the reaction, allows the enzyme to cleave peptide bonds with high efficiency. This mechanism is a classic example of convergent evolution, as it is found in many different, evolutionarily unrelated proteases.

..

In biology, H+ ions (protons) are donated by a wide range of molecules, primarily those containing polar covalent bonds to electronegative atoms like oxygen and nitrogen, as well as certain specialized carbon-hydrogen bonds.

The most common biological H+ donors are acids, water, and specific functional groups within macromolecules.

Major H+ Donors in Biological Systems Small Molecules & Ions Water:

The most abundant H+ donor and acceptor, constantly exchanging protons in a flicker of reactions essential for life.

Organic Acids:

Carboxylic acids:

Found in amino acids (aspartic acid, glutamic acid) and metabolic intermediates like pyruvate, lactate, and citrate.

Phosphates:

Key components of nucleotides (ATP, NADP+) and the DNA/RNA backbone; their hydroxyl groups readily donate protons.

Buffer Systems:

Carbonic acid:

Part of the primary buffer system in the blood, dissociating into bicarbonate and a proton.

Ammonium ions:

Can act as H+ donors in various contexts.

Macromolecules (Proteins, Nucleic Acids, etc.) Proteins and Amino Acids:

Charged amino acid side chains:

The protonated forms of arginine (argininium), lysine (lysinium), and histidine (histidinium) are the strongest H-bond donors.

Polar amino acid side chains:

The hydroxyl groups of serine, threonine, and tyrosine, and the amide groups of asparagine and glutamine, also function as donors.

Peptide backbone:

The amide groups in the protein backbone are crucial donors for stabilizing secondary structures like alpha-helices and beta-sheets.

Nucleic Acids (DNA and RNA):

Nitrogenous bases:

Specific amine and ring nitrogen groups in adenine, guanine, cytosine, and thymine/uracil serve as donors in base pairing (Watson-Crick hydrogen bonding).

Sugar backbone:

Hydroxyl groups on the ribose or deoxyribose sugars are involved in intramolecular hydrogen bonds that stabilize structure.

C-H groups:

Even certain polarized C-H groups, particularly those adjacent to electronegative atoms within proteins and nucleic acids, can act as weak, but structurally significant, H-bond donors.