Aldehyde dehydrogenase, dimeric NADP-preferring is an

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

that in humans is encoded by the ''ALDH3A1''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

Aldehyde dehydrogenase Aldehyde dehydrogenases () are a group of enzymes that catalyse the oxidation of aldehydes. They convert aldehydes (R–C(=O)) to carboxylic acids (R–C(=O)). The oxygen comes from a water molecule. To date, nineteen ALDH genes have ...

s oxidize various

aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

s to the corresponding acids. They are involved in the detoxification of alcohol-derived acetaldehyde and in the metabolism of

corticosteroid Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex of vertebrates, as well as the synthetic analogues of these hormones. Two main classes of corticosteroids, glucocorticoids and mineralocorticoids, are invo ...

s, biogenic amines, neurotransmitters, and lipid peroxidation. The enzyme encoded by this gene forms a cytoplasmic homodimer that preferentially oxidizes aromatic aldehyde substrates. The gene is located within the

Smith–Magenis syndrome Smith–Magenis syndrome (SMS), also known as 17p-microdeletion syndrome, is a microdeletion syndrome characterized by an abnormality in the short (p) arm of chromosome 17. It has features including intellectual disability, facial abnormalities, ...

region on chromosome 17. ALDH3A1 expression is notably high in the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

of mammalian species, comprising from 5 to 50% of soluble protein content, but is almost absent from the cornea of other vertebrates.

Structure and mechanism

ALDH3A1 is a homodimer consisting of alpha helices (43.8%), beta sheets (4.2%), p-loop turns (28.2%) and random coils (23.8%). The

catalytic residue In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding si ...

–Cys244—is located on an active site that contains a

Rossmann fold The Rossmann fold is a tertiary fold found in proteins that bind nucleotides, such as enzyme cofactors FAD, NAD+, and NADP+. This fold is composed of alternating beta strands and alpha helical segments where the beta strands are hydrogen bond ...

that binds the enzyme's cofactor, NAD(P)+. 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. 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.

Involvement in lipid peroxidation

Stoichiometry of ALDH3A1 aldehyde metabolism

Electronic excitations of alkene and aromatic functional groups allow certain

nucleic acids Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic a ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...

s and organic molecules to absorb

ultraviolet radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

(UVR). Moderate UVR exposure oxidizes specific proteins that eventually serve as signaling agents for an array of

metabolic Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the ...

and inflammatory pathways. 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 In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...

(ROS) that are implicated in many degradation pathways. In the case of

lipid peroxidation Lipid peroxidation, or lipid oxidation, is a complex chemical process that leads to oxidative degradation of lipids, resulting in the formation of peroxide and hydroperoxide derivatives.{{Cite journal , last1=Ayala , first1=Antonio , last2=Muñoz ...

, ROS react with

polyunsaturated fatty acids In biochemistry and nutrition, a polyunsaturated fat is a fat that contains a polyunsaturated fatty acid (abbreviated PUFA), which is a subclass of fatty acid characterized by a backbone with two or more carbon–carbon double bonds. Some polyunsa ...

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

s, which, owing to their stability and ability to react cellular nucleophiles, are both

cytotoxic Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are toxic metals, toxic chemicals, microbe neurotoxins, radiation particles and even specific neurotransmitters when the system is out of balance. Also some types of dr ...

and

genotoxic Genotoxicity is the property of chemical agents that damage the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, but some genotoxic s ...

in nature. ALDH3A1 plays a critical role in the metabolism of these aldehydes to their corresponding

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s in mammalian cornea and saliva.

4-Hydroxynonenal 4-Hydroxynonenal, or 4-hydroxy-2E-nonenal or 4-hydroxy-2-nonenal or 4-HNE or HNE, (), is an α,β-unsaturated hydroxyalkenal that is produced by lipid peroxidation in cells. 4-HNE is the primary α,β-unsaturated hydroxyalkenal formed in this pr ...

(4HNE)—which ALDH3A1 metabolizes with V_max of 27,754 moles NADPH/min•mg and an apparent K_m of 362 micromolar —is the most abundant aldehyde produced in the LPO of

arachidonic acid Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega−6 fatty acid 20:4(ω−6), or 20:4(5,8,11,14). It is a precursor in the formation of leukotrienes, prostaglandins, and thromboxanes. Together with omega−3 fatty acids an ...

and

linoleic acid Linoleic acid (LA) is an organic compound with the formula . Both alkene groups () are ''cis''. It is a fatty acid sometimes denoted 18:2 (n−6) or 18:2 ''cis''-9,12. A linoleate is a salt or ester of this acid. Linoleic acid is a polyunsat ...

. Its stability and multiple sites of reactivity (carbon-carbon double bond, hydroxyl group, and carbonyl) make 4HNE a potent inhibitor of

cellular growth Cellular may refer to: *Cellular automaton, a model in discrete mathematics *Cell biology, the evaluation of cells work and more * ''Cellular'' (film), a 2004 movie *Cellular frequencies, assigned to networks operating in cellular RF bands *Cellu ...

, enzyme activities, calcium sequestration, and

protein synthesis Protein biosynthesis, or protein synthesis, is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critica ...

. It is also involved in the consumption of

glutathione Glutathione (GSH, ) is an organic compound with the chemical formula . It is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources ...

and the alteration of

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

and

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

Role in the cornea

Residues that degrade under UVR exposure

ALDH3A1 comprises approximately 10-40% of the water-soluble protein in the mammalian

. Direct exposure to UVR and

molecular oxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are: * Atomic ...

, 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 stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...

es. 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

(GSH); (3) protects the corneal cells from 4-HNE induced

apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

; (4) reduces consumption of GSH by relieving 4HNE GSH adducts; (5) and relieves 4-HNE's inhibition of the 20S

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

activity.

Suicide response to UVR

However, only a fraction of the total concentration of ALDH3A1 in the cornea is used for metabolizing

s. This observation has sparked multiple investigations of ALDH3A1's role beyond aldehyde metabolism. 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” that reduces damage to other proteins of the cornea 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 Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

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 Enzyme assays are laboratory methods for measuring enzymatic activity. They are vital for the study of enzyme kinetics and enzyme inhibition. Enzyme units The quantity or concentration of an enzyme can be expressed in molar amounts, as with any ...

Peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

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 Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other non-essential amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine play ...

and

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromat ...

). These amino acid residues degrade under oxidative stress, leading to the formation of non-reducible cross-links that stabilize the soluble aggregates. Tryptophan for instance is doubly oxidized to generate ROSs such as H₂O₂, which elicit further oxidation and adduction. 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. Cataract in human eye

Consequences of ALDH3A1 deficiency

Further clarification of ALDH3A1's role in the cornea has been provided by gene-knockout studies in which genes encoding ALDH3A1 were removed from the mice genome. It was found that ALDH3A1-null mice exhibited lower proteasome activity, higher rates of protein degradation/oxidation, and higher GSH, 4HNE and

malondialdehyde Malondialdehyde belong to the class of β-dicarbonyls. A colorless solid, malondialdehyde is a highly reactive compound that occurs as the enol. It is a physiological metabolite, and a marker for oxidative stress. Structure and synthesis Malon ...

protein adduct levels—all of which contributed to the development of

cataracts A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision of the eye. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colours, blurry or double vision, halos around ligh ...

and opacities in the subscapular regions of the cornea within one month of age. These observations on ALDH3A1-null mice reaffirm that ALDH3A1's role extends beyond enzymatic metabolism; encompassing functions in maintenance of the structural integrity and transparency of the cornea.

References

External links

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