Aldehyde dehydrogenase 3 family, member A1

ALDH3A1 bound to NAD+

ALDH3A1 is a homodimer consisting of alpha helices (43.8%), beta sheets (4.2%), p-loop turns (28.2%) and random coils (23.8%).[9] The catalytic residue–Cys244—is located on an active site that contains a Rossmann fold that binds the enzyme's cofactor, NAD(P)+.[10]

ALDH3A1's catalytic mechanism mirrors that of other enzymes of the aldehyde dehydrogenase family. The sulfur atom of Cys244 attacks the carbonyl of the aldehyde substrate in a nucleophilic attack that releases a hydride ion. The hydride ion is accepted by the NAD(P)+ bound to the Rossmann fold. Unique interactions between the cofactor and the Rossmann fold facilitate an isomerization of the enzyme that releases the cofactor while maintaining the integrity of the active site.[11] A water molecule enters the active site and is subsequently activated by a glutamate residue. The activated water then attacks the thioester enzyme-substrate complex in nucleophilic reaction that regenerates the free enzyme, and releases the corresponding carboxylic acid.

Stoichiometric equation representing the metabolism of an aldehyde substrate by ALDH3A1 using NADP+ as a cofactor

Electronic excitations of alkene and aromatic functional groups allow certain nucleic acids, proteins, fatty acids and organic molecules to absorb ultraviolet radiation (UVR). Moderate UVR exposure oxidizes specific proteins that eventually serve as signaling agents for an array of metabolic and inflammatory pathways.[9] Overexposure to UVR, on the other hand, can be detrimental to the tissue. In the presence of molecular oxygen, UVR leads to the formation of reactive oxygen species (ROS) that are implicated in many degradation pathways.[12] In the case of lipid peroxidation, ROS react with polyunsaturated fatty acids situated in the lipid bilayer of the cell membrane to produce lipid radicals. These lipid radicals propagate, further damaging the lipid bilayer and producing lipid hydroperoxides. The eventual degradation of lipid hydroperoxides releases a wide variety of aldehydes, which, owing to their stability and ability to react cellular nucleophiles,[12] are both cytotoxic and genotoxic in nature. ALDH3A1 plays a critical role in the metabolism of these aldehydes to their corresponding carboxylic acids in mammalian cornea and saliva. 4-Hydroxynonenal (4HNE)—which ALDH3A1 metabolizes with V_max of 27,754 moles NADPH/min•mg and an apparent K_m of 362 micromolar[9] —is the most abundant aldehyde produced in the LPO of arachidonic acid and linoleic acid.[13][14] Its stability and multiple sites of reactivity (carbon-carbon double bond, hydroxyl group, and carbonyl) make 4HNE a potent inhibitor of cellular growth, enzyme activities, calcium sequestration, and protein synthesis. It is also involved in the consumption of glutathione and the alteration of signal transduction and gene expression.[15][16][17][18][19]

Structure showing the amino acid residues that degrade under UVR exposure. Shown are Tryptophan-81, Cysteine-223, Cysteine-229, Tryptophan-234, Methionine-295 and Methionine-366

ALDH3A1 comprises approximately 10-40% of the water-soluble protein in the mammalian cornea.[20][21] Direct exposure to UVR and molecular oxygen, make the cornea susceptible to ROS and 4HNE. Studies in which rabbits were transfected with genes that allow them to overexpress human ALDH3A1 in their corneal stromal fibroblasts document ALDH3A1's most critical function is to protect the cornea from oxidative stresses. In the cornea ALDH3A1: (1) prevents the formation of 4-HNE protein adducts that would impeded proteins’ function; (2) is more effective at metabolizing 4-HNE than other comparable agents such as glutathione (GSH); (3) protects the corneal cells from 4-HNE induced apoptosis; (4) reduces consumption of GSH by relieving 4HNE GSH adducts; (5) and relieves 4-HNE's inhibition of the 20S protease activity.[22]

Suicide response to UVR

However, only a fraction of the total concentration of ALDH3A1 in the cornea is used for metabolizing aldehydes. This observation has sparked multiple investigations of ALDH3A1's role beyond aldehyde metabolism.[23] Although the full scope of ALDH3A1's function is yet to be firmly established, there is strong evidence suggesting that ALDH3A1 serves to maintain the cellular redox balance as well as the structural integrity and transparency of the cornea. One study elucidates that ALDH3A1 not only indirectly protects the cornea from UVR induced oxidative stress by metabolizing aldehydes, but also protects the tissue directly, by competitively absorbing UVR in a “suicide response”[8] that reduces damage to other proteins of the cornea[9] In fact, 50% percent of the UVR that the cornea is exposed to is absorbed by ADLH3A1. ALDH3A1's absorption of UVR oxidizes several key amino acid residues, leading to conformational changes that convert the alpha and beta sheets into random coils. These conformational changes ultimately relieve the dimer structure. This loss of secondary and tertiary structure leads to protein aggregation and complete loss of enzymatic activity.[9] Peptide mapping and spectroscopic experiments reveal that the loss of activity is not a result of Cys244 oxidation (which, together with the active site, remains intact during photo-excitation), but instead, due to the degradation of other key amino residues (most notably methionine and tryptophan). These amino acid residues degrade under oxidative stress, leading to the formation of non-reducible cross-links that stabilize the soluble aggregates.[9] Tryptophan for instance is doubly oxidized to generate ROSs such as H₂O₂, which elicit further oxidation and adduction.[24] Nevertheless, the abundance of ALDH3A1 in the cornea ensures that this suicide response neither impedes with aldehyde metabolism nor leads to the formation of insoluble aggregates that would affect the transparency of the cornea.[25]

Cataract in human eye

• Human ALDH3A1 genome location and ALDH3A1 gene details page in the UCSC Genome Browser.
• Overview of all the structural information available in the PDB for UniProt: P30838 (Aldehyde dehydrogenase, dimeric NADP-preferring) at the PDBe-KB.