What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Histone Deacetylase: Function, Mechanism, and Role in Gene Regulation

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Histone Deacetylases

Histone deacetylases (HDACs) are a family of enzymes that catalyze the removal of acetyl groups from ε-amino groups of lysine residues on histone proteins. This enzymatic activity is the biochemical counterpart to histone acetylation, and together these opposing reactions constitute a dynamic regulatory system that controls chromatin structure and gene expression. By removing acetyl groups, HDACs promote a more compact chromatin conformation that is generally associated with transcriptional repression.

What Are Histone Deacetylases?

Histone deacetylases are hydrolases that cleave the amide bond linking an acetyl moiety to the side-chain nitrogen of lysine residues. The reaction produces a deacetylated lysine and a free acetate molecule. While the name emphasizes histones as substrates, HDACs also deacetylate numerous non-histone proteins, including transcription factors, signaling molecules, and metabolic enzymes. The human genome encodes 18 HDAC enzymes, divided into four classes based on sequence homology, structure, and catalytic mechanism.

The fundamental importance of HDACs lies in their ability to influence gene expression at the level of chromatin. DNA in eukaryotic cells is wrapped around histone octamers to form nucleosomes, the basic repeating unit of chromatin. Each histone octamer contains two copies each of H2A, H2B, H3, and H4. The N-terminal tails of these histones protrude from the nucleosome core and are subject to numerous post-translational modifications, including acetylation, methylation, phosphorylation, and ubiquitination. Acetylation of lysine residues neutralizes the positive charge on histone tails, weakening their electrostatic interaction with negatively charged DNA. This promotes a more open chromatin structure that permits access of transcriptional machinery to DNA. HDACs reverse this modification, restoring positive charge and favoring chromatin compaction.

Historical Discovery

The first histone deacetylase activity was purified from calf thymus in the late 1970s by Vincent Allfrey and colleagues, who had earlier discovered histone acetylation itself. However, the molecular identity of HDACs remained elusive until 1996, when Stuart Schreiber's laboratory identified the first mammalian HDAC (now called HDAC1) through affinity purification using a trapoxin-based inhibitor. In the same year, David Allis's group cloned the first histone acetyltransferase (Gcn5) from yeast. These parallel discoveries established the framework for understanding reversible histone acetylation as a central mechanism of gene regulation. The subsequent identification of the yeast transcriptional repressor Rpd3 as an HDAC homolog cemented the connection between deacetylation and transcriptional silencing.

The Role of Histone Acetylation and Deacetylation

The acetylation state of histone lysine residues reflects a dynamic equilibrium set by the opposing activities of histone acetyltransferases (HATs) and histone deacetylases. This balance is not static; it shifts rapidly in response to developmental cues, environmental signals, and cellular stress.

Histone Acetyltransferases (HATs) vs. HDACs

Histone acetyltransferases catalyze the transfer of an acetyl group from acetyl-coenzyme A (acetyl-CoA) to the ε-amino group of specific lysine residues on histone tails. This reaction neutralizes the lysine's positive charge and is generally associated with transcriptional activation. HATs are typically found within large multiprotein complexes that are recruited to enhancers and promoters by sequence-specific transcription factors.

HDACs oppose this activity. They remove acetyl groups, restoring the positive charge on lysine and promoting chromatin condensation. The balance between HAT and HDAC activity at any given genomic locus determines the local acetylation level and, consequently, the transcriptional state. This antagonism is not merely biochemical; HATs and HDACs are often physically recruited to the same promoters by different transcription factors, creating a dynamic tug-of-war that fine-tunes gene expression.

Acetylation and Chromatin Structure

The biochemical consequences of histone acetylation extend beyond charge neutralization. Acetylated lysine residues serve as docking sites for bromodomain-containing proteins, which recognize acetylated lysine and recruit additional chromatin remodeling complexes, transcriptional coactivators, and the basal transcriptional machinery. Conversely, deacetylated histones are recognized by chromodomain-containing proteins that promote transcriptional silencing.

At the structural level, hyperacetylation of histone tails reduces their affinity for DNA, destabilizing the histone nucleosome and facilitating the sliding or eviction of nucleosomes by ATP-dependent chromatin remodelers. This creates accessible regulatory regions where transcription factors can bind. Deacetylation by HDACs stabilizes nucleosome-DNA interactions, promoting a closed chromatin state that occludes transcription factor binding sites. The net effect is that HDAC activity generally correlates with transcriptional repression, although as discussed later, this is an oversimplification.

Mechanism of Histone Deacetylation

The catalytic mechanism of HDACs depends on the enzyme class. Zinc-dependent HDACs (Classes I, II, and IV) use a metal-activated water molecule to hydrolyze the acetyl-lysine amide bond. The Class III sirtuins use NAD⁺ as a cofactor in a fundamentally different reaction.

Catalytic Mechanism

The active site of zinc-dependent HDACs contains a conserved histidine-aspartate charge-relay system and a zinc ion coordinated by two aspartate and one histidine residue. The reaction proceeds through the following steps:

The acetyl carbonyl oxygen coordinates to the active-site zinc ion, polarizing the carbonyl group and making the carbon more electrophilic.
A water molecule, activated by the histidine-aspartate charge-relay system, performs a nucleophilic attack on the carbonyl carbon.
A tetrahedral oxyanion intermediate is formed, stabilized by coordination to the zinc ion and hydrogen bonding with active-site residues.
The intermediate collapses, breaking the amide bond and releasing the deacetylated lysine and acetate.

The catalytic rate of HDACs is relatively slow compared to many enzymes, with turnover numbers in the range of 1–10 s⁻¹. This reflects the challenge of hydrolyzing a stable amide bond in a protein substrate. The active-site channel of HDACs is a narrow, approximately 8 Å deep tunnel that accommodates the lysine side chain but excludes larger peptide substrates, explaining why HDACs can deacetylate internal lysine residues without processively degrading the protein.

Zinc-Dependent HDACs

The zinc-dependent HDACs include Class I (HDAC1, 2, 3, 8), Class II (HDAC4, 5, 6, 7, 9, 10), and Class IV (HDAC11). These enzymes share a conserved deacetylase domain of approximately 350 amino acids. The zinc ion is essential for catalysis; removal of zinc by chelating agents such as EDTA abolishes activity. The catalytic mechanism is conserved from bacteria to humans, reflecting the ancient origin of this enzyme family.

Class I HDACs are primarily nuclear enzymes with ubiquitous expression. HDAC1 and HDAC2 are found in the nucleosome remodeling and deacetylase (NuRD), Sin3, and CoREST complexes. HDAC3 is associated with the SMRT/N-CoR complexes. Class II HDACs (HDAC4, 5, 7, 9) can shuttle between the nucleus and cytoplasm and show more tissue-restricted expression, particularly in muscle, heart, and brain. HDAC6 is predominantly cytoplasmic and deacetylates tubulin and heat shock protein 90. HDAC10 shares homology with Class II but has an unusual second catalytic domain. HDAC11, the sole Class IV member, has a conserved catalytic domain but is structurally distinct.

NAD⁺-Dependent Sirtuins

The Class III HDACs, or sirtuins, require nicotinamide adenine dinucleotide (NAD⁺) as a cofactor. The reaction is fundamentally different from zinc-dependent deacetylation:

NAD⁺ binds to the sirtuin active site, and the acetyl-lysine substrate binds adjacent to it.
The nicotinamide glycosidic bond of NAD⁺ is cleaved, releasing nicotinamide and generating an ADP-ribose-oxocarbenium intermediate.
The acetyl group from the lysine is transferred to the 2'-OH of the ADP-ribose, forming a 1'-O-acetyl-ADP-ribose intermediate.
This intermediate undergoes intramolecular transesterification to form 2'-O-acetyl-ADP-ribose.
The deacetylated lysine is released, and the 2'-O-acetyl-ADP-ribose dissociates from the enzyme.

This mechanism couples deacetylation to NAD⁺ hydrolysis, making sirtuin activity sensitive to the cellular energy status. The seven human sirtuins (SIRT1–7) have distinct subcellular localizations: SIRT1, SIRT6, and SIRT7 are nuclear; SIRT2 is cytoplasmic; SIRT3, SIRT4, and SIRT5 are mitochondrial. SIRT1 is the best-characterized sirtuin and deacetylates both histone and non-histone substrates, including p53, FOXO transcription factors, and PGC-1α.

Classification of Histone Deacetylases

The 18 human HDACs are classified into four groups based on sequence homology to yeast orthologs and cofactor dependence. This classification is clinically relevant because different HDAC inhibitors have different isoform selectivity profiles.

Class I, II, and IV HDACs

Class	Members	Cofactor	Subcellular Localization	Yeast Ortholog
I	HDAC1, 2, 3, 8	Zn²⁺	Nuclear	Rpd3
IIa	HDAC4, 5, 7, 9	Zn²⁺	Nuclear/cytoplasmic	Hda1
IIb	HDAC6, 10	Zn²⁺	Predominantly cytoplasmic	Hda1
III	SIRT1–7	NAD⁺	Nuclear, cytoplasmic, mitochondrial	Sir2
IV	HDAC11	Zn²⁺	Nuclear/cytoplasmic	—

Class I HDACs are expressed ubiquitously and are predominantly nuclear. They are relatively small proteins (approximately 400–500 amino acids) with a single catalytic domain. Class II HDACs are larger (approximately 1000 amino acids) and contain extended N-terminal domains that mediate protein-protein interactions and subcellular localization. Class IIa HDACs (HDAC4, 5, 7, 9) have a unique feature: their catalytic domains contain a tyrosine residue in place of the conserved histidine that is critical for catalysis in Class I enzymes. This substitution reduces their intrinsic deacetylase activity by approximately 1000-fold compared to Class I HDACs, suggesting that Class IIa enzymes may primarily function as scaffolds that recruit other HDACs, particularly HDAC3, to target loci.

Class III Sirtuins

Sirtuins are structurally unrelated to the zinc-dependent HDACs. They share a conserved catalytic core of approximately 275 amino acids that binds NAD⁺ and the acetyl-lysine substrate. Sirtuins have diverse biological functions beyond histone deacetylation, including regulation of metabolism, stress responses, and aging. SIRT1 deacetylates histone H4K16 and H3K9, as well as numerous non-histone substrates. SIRT6 deacetylates H3K9 and H3K56 and also has mono-ADP-ribosyltransferase activity. SIRT7 is associated with RNA polymerase I and regulates ribosomal DNA transcription.

Regulation of Histone Deacetylase Activity

HDAC activity is not constitutive; it is tightly regulated by multiple mechanisms that allow cells to modulate deacetylase function in response to physiological signals.

Phosphorylation and Ubiquitination

Phosphorylation is a major regulatory mechanism for Class IIa HDACs. HDAC4, 5, 7, and 9 contain conserved serine residues that are phosphorylated by calcium/calmodulin-dependent protein kinase II (CaMKII) and protein kinase D. Phosphorylation creates binding sites for 14-3-3 proteins, which sequester the HDACs in the cytoplasm, preventing their nuclear function. Dephosphorylation by protein phosphatase 2A (PP2A) releases the 14-3-3 interaction, allowing nuclear import and transcriptional repression. This phosphorylation-dependent shuttling is particularly important in muscle differentiation and neuronal activity-dependent gene expression.

Class I HDACs are also phosphorylated. HDAC1 and HDAC2 are phosphorylated by casein kinase 2 (CK2) at serine residues near the C-terminus, which is required for their catalytic activity and assembly into multiprotein complexes. Ubiquitination regulates HDAC protein stability. HDAC1 and HDAC2 are ubiquitinated and degraded by the proteasome in response to specific signals, and deubiquitinases such as USP7 can reverse this modification.

Interaction with Co-repressor Complexes

Most HDACs do not act alone; they are recruited to chromatin as components of large multiprotein co-repressor complexes. HDAC1 and HDAC2 are found in at least three distinct complexes: Sin3, NuRD, and CoREST. Each complex contains unique subunits that provide targeting specificity and additional enzymatic activities. The NuRD complex, for example, contains the ATP-dependent chromatin remodeler CHD4, which couples histone deacetylation to nucleosome remodeling. HDAC3 is found in the SMRT/N-CoR complexes, which also contain the kinase TBL1 and the ubiquitin-conjugating enzyme UbcH5.

These complexes are recruited to specific genomic loci by sequence-specific DNA-binding transcription factors. For example, the tumor suppressor p53 recruits the Sin3/HDAC complex to repress genes involved in cell cycle progression, while nuclear hormone receptors recruit SMRT/N-CoR/HDAC3 complexes to repress target genes in the absence of ligand. The association of HDACs with these complexes is dynamic and regulated by post-translational modifications of both the HDACs and the complex components.

Biological Functions of Histone Deacetylases

HDACs participate in nearly every aspect of genome function, from transcriptional regulation to DNA repair and chromosome segregation. Their diverse biological roles reflect both their histone substrates and their growing list of non-histone targets.

Gene Silencing

The canonical function of HDACs is transcriptional repression. By deacetylating histone lysine residues, HDACs promote chromatin compaction and prevent access of transcription factors and RNA polymerase to DNA. This is particularly important at developmental genes that must be maintained in a silent state, at imprinted loci, and at repetitive elements that must remain transcriptionally inactive.

HDACs also cooperate with histone methylation to establish and maintain silenced chromatin. Deacetylation of H3K9 creates a favorable substrate for the methyltransferase SUV39H1, which methylates H3K9 to create a binding site for heterochromatin protein 1 (HP1). This establishes a positive feedback loop that propagates heterochromatin. The interplay between deacetylation and methylation is a key feature of the histone code, the hypothesis that combinations of histone modifications encode regulatory information.

Cell Cycle and Apoptosis

HDACs regulate cell cycle progression through multiple mechanisms. HDAC1 and HDAC2 are required for normal S-phase progression, and their deletion causes cell cycle arrest. HDAC3 is essential for the G1/S transition, partly through regulation of cyclin E expression. HDACs also deacetylate non-histone proteins involved in cell cycle control, including the retinoblastoma protein (Rb) and the p53 tumor suppressor.

Deacetylation of p53 at lysine residues K320, K373, and K382 reduces its transcriptional activity and promotes its degradation. HDAC inhibitors therefore stabilize p53 and induce apoptosis in cancer cells. HDACs also regulate apoptosis through deacetylation of pro-apoptotic and anti-apoptotic proteins. For example, HDAC6 deacetylates the pro-apoptotic protein Bax, promoting its activation, while HDAC1 deacetylates the survival factor STAT3, modulating its transcriptional activity.

HDACs in Disease

Given their central role in gene regulation, it is not surprising that HDAC dysfunction is associated with numerous diseases. In cancer, HDACs are frequently overexpressed or aberrantly recruited to tumor suppressor genes, contributing to their silencing. HDAC1 is overexpressed in gastric, colorectal, and prostate cancers, and high HDAC expression often correlates with poor prognosis. Chromosomal translocations that fuse HDACs or HDAC-associated proteins to transcription factors are found in specific leukemias and lymphomas.

HDACs are also implicated in neurological disorders. HDAC2 expression is elevated in Alzheimer's disease, and its overexpression in mouse models impairs synaptic plasticity and memory formation. HDAC4 mutations cause brachydactyly-mental retardation syndrome, and HDAC6 dysfunction is associated with neurodegenerative conditions. Inflammatory diseases, metabolic disorders, and viral infections also involve HDAC activity, making these enzymes attractive therapeutic targets.

Methods to Study Histone Deacetylases

Studying HDACs requires a combination of biochemical, cellular, and genomic approaches. Each method provides complementary information about HDAC activity, localization, and function.

Activity Assays

HDAC activity can be measured using fluorogenic substrates. The most common substrate is Boc-Lys(Ac)-AMC, a peptide containing an acetylated lysine linked to a fluorophore. Deacetylation by HDACs unmasks the lysine, which is then cleaved by trypsin in a second step to release the fluorescent AMC group. The fluorescence is proportional to HDAC activity. This assay is performed in 50 mM Tris-HCl (pH 8.0), 137 mM NaCl, 2.7 mM KCl, 1 mM MgCl₂, and 1 mg/mL bovine serum albumin, with 50 µM substrate and 0.5–2 µg of enzyme or cell extract. Reactions are incubated at 37°C for 30–60 minutes, followed by addition of trypsin (0.5 mg/mL) and a further 15–30 minute incubation before fluorescence measurement at 360 nm excitation and 460 nm emission.

For sirtuins, the substrate is typically a peptide containing acetyl-lysine conjugated to a fluorophore, and the reaction requires NAD⁺ (typically 500 µM). Sirtuin activity can also be measured by monitoring NAD⁺ consumption or the production of 2'-O-acetyl-ADP-ribose. Radiometric assays using [³H]-acetylated histones are more sensitive but require specialized equipment and handling.

Chromatin Immunoprecipitation (ChIP)

Chromatin immunoprecipitation is the standard method for determining where HDACs bind in the genome and what histone modifications they control. The basic protocol involves:

Cross-linking proteins to DNA using 1% formaldehyde for 10 minutes at room temperature.
Quenching the cross-linking reaction with 125 mM glycine.
Lysing cells and shearing chromatin by sonication to fragments of 200–600 base pairs.
Immunoprecipitating with an antibody against the HDAC of interest or against a specific histone acetylation mark (e.g., H3K9ac or H4K16ac).
Reversing cross-links by heating at 65°C for 4–6 hours in the presence of proteinase K.
Purifying the DNA and analyzing by quantitative PCR (qPCR) or high-throughput sequencing (ChIP-seq).

ChIP-seq provides genome-wide maps of HDAC occupancy and histone acetylation patterns. A typical ChIP-seq experiment requires 10⁶–10⁷ cells and produces 20–50 million sequencing reads per sample. The data reveal that HDACs are not uniformly distributed across the genome but are enriched at promoters, enhancers, and heterochromatic regions.

HDAC Inhibitors as Tools

HDAC inhibitors are invaluable research tools for probing HDAC function. The broad-spectrum inhibitors trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA, vorinostat) inhibit Class I, II, and IV HDACs at nanomolar concentrations. TSA is typically used at 0.1–1 µM for 6–24 hours in cell culture. Sodium butyrate is a weaker inhibitor (millimolar range) that is often used at 1–5 mM. The sirtuin inhibitor nicotinamide is used at 1–10 mM, and more selective sirtuin inhibitors such as EX-527 (for SIRT1) are available.

Treatment of cells with HDAC inhibitors causes global hyperacetylation of histones and changes in the expression of 2–10% of genes. The transcriptional effects are highly context-dependent: roughly equal numbers of genes are up- and down-regulated, reflecting the fact that HDAC inhibition affects both direct and indirect targets.

Histone Deacetylase Inhibitors and Therapeutic Applications

The development of HDAC inhibitors as therapeutic agents has been one of the most active areas of translational research in epigenetics. Several HDAC inhibitors are now approved for clinical use, primarily in oncology.

Classes of HDAC Inhibitors

HDAC inhibitors are classified by their chemical structure:

Hydroxamates: TSA, SAHA (vorinostat), belinostat, panobinostat. These are broad-spectrum inhibitors that chelate the active-site zinc ion. SAHA was the first HDAC inhibitor approved by the FDA (2006) for cutaneous T-cell lymphoma.
Cyclic peptides: Romidepsin (FK228), a prodrug that is reduced intracellularly to release a zinc-binding thiol. Approved for cutaneous T-cell lymphoma and peripheral T-cell lymphoma.
Benzamides: Entinostat (MS-275), mocetinostat. These are selective for Class I HDACs and have longer half-lives than hydroxamates.
Short-chain fatty acids: Sodium butyrate, valproic acid. These are weak inhibitors (millimolar IC₅₀) but have been used clinically for decades, particularly valproic acid as an anticonvulsant.
Sirtuin inhibitors: Nicotinamide, EX-527, sirtinol. These are less developed clinically but are being explored for cancer and metabolic diseases.

Clinical Use and Side Effects

HDAC inhibitors have demonstrated efficacy in hematological malignancies, particularly T-cell lymphomas. Vorinostat and romidepsin are approved for cutaneous T-cell lymphoma, and panobinostat is approved for multiple myeloma in combination with bortezomib and dexamethasone. The mechanisms of action in cancer are multifaceted: HDAC inhibitors reactivate silenced tumor suppressor genes, induce cell cycle arrest and apoptosis, generate reactive oxygen species, and enhance immunogenicity of tumor cells.

The clinical use of HDAC inhibitors is limited by significant side effects, including fatigue, nausea, diarrhea, thrombocytopenia, and QT interval prolongation (a cardiac arrhythmia risk). These toxicities reflect the broad biological roles of HDACs and the lack of isoform selectivity of most inhibitors. The therapeutic window is narrow, and not all patients respond. Current research focuses on developing isoform-selective inhibitors and identifying biomarkers that predict response.

Common Pitfalls and Misconceptions

Several misconceptions about HDACs are common among students and even researchers. Understanding these nuances is essential for accurate interpretation of experimental data.

Non-Histone Substrates

The name "histone deacetylase" is misleading because HDACs deacetylate hundreds of non-histone proteins. The acetylated lysine is a common post-translational modification found on transcription factors (p53, NF-κB, STAT3), cytoskeletal proteins (α-tubulin), chaperones (Hsp90), and metabolic enzymes. In fact, the number of known non-histone substrates exceeds the number of histone substrates. HDAC6, for example, is primarily a cytoplasmic enzyme that deacetylates α-tubulin and Hsp90, with little activity toward histones. When interpreting HDAC inhibitor effects, one must consider both histone and non-histone consequences.

Context-Dependent Effects

HDAC inhibition does not always lead to gene activation. While deacetylation is generally associated with repression, HDACs can also repress repressors, leading to indirect gene activation. Moreover, HDACs are recruited to active genes where they reset acetylation marks after transcription, and their inhibition can paradoxically reduce expression of some genes. The transcriptional response to HDAC inhibition is highly cell-type specific and depends on the balance of activators and repressors at each locus. Genome-wide studies show that HDAC inhibitors up-regulate and down-regulate roughly equal numbers of genes.

Isoform Specificity

Not all HDACs are functionally equivalent. Class I HDACs are potent deacetylases with broad expression, while Class IIa HDACs have weak catalytic activity and may function primarily as scaffolds. Sirtuins have different cofactor requirements and substrates. The common practice of treating all HDACs as a single entity obscures important functional differences. Isoform-specific functions are revealed by genetic studies: knockout of HDAC1 is embryonic lethal, while knockout of HDAC6 produces viable mice with only mild phenotypes. This functional diversity has important implications for drug development.

Frequently Asked Questions

What is histone deacetylase?

Histone deacetylase (HDAC) is an enzyme that removes acetyl groups from lysine residues on histone proteins. This reaction reverses histone acetylation, promotes chromatin compaction, and is generally associated with transcriptional repression. The human genome encodes 18 HDACs divided into four classes based on sequence homology and catalytic mechanism.

What is the function of histone deacetylase?

The primary function of HDACs is to regulate gene expression by controlling the acetylation state of histones. Deacetylation restores the positive charge on histone lysine residues, strengthening histone-DNA interactions and promoting chromatin condensation. HDACs also deacetylate non-histone proteins, thereby regulating their activity, stability, and interactions.

What is the mechanism of histone deacetylase?

Zinc-dependent HDACs (Classes I, II, IV) use a zinc ion to polarize the acetyl carbonyl group, activating a water molecule for nucleophilic attack on the carbonyl carbon. This hydrolyzes the amide bond, releasing acetate and the deacetylated lysine. Sirtuins (Class III) use NAD⁺ as a cofactor, cleaving the glycosidic bond of NAD⁺ and transferring the acetyl group to ADP-ribose.

How do histone deacetylases affect gene expression?

HDACs affect gene expression primarily by removing acetyl groups from histone tails, which promotes chromatin compaction and reduces accessibility of DNA to transcription factors and RNA polymerase. HDACs are recruited to specific genes by co-repressor complexes and sequence-specific transcription factors, allowing targeted regulation of gene expression.

What are the classes of histone deacetylases?

The four classes are: Class I (HDAC1, 2, 3, 8), Class II (HDAC4, 5, 6, 7, 9, 10), Class III (SIRT1–7), and Class IV (HDAC11). Classes I, II, and IV are zinc-dependent; Class III sirtuins require NAD⁺.

What is the difference between HAT and HDAC?

HATs (histone acetyltransferases) add acetyl groups to histone lysine residues, promoting open chromatin and transcriptional activation. HDACs remove acetyl groups, promoting closed chromatin and transcriptional repression. These enzymes act antagonistically to dynamically regulate gene expression.

What are HDAC inhibitors?

HDAC inhibitors are compounds that block HDAC enzymatic activity. They include hydroxamates (TSA, SAHA), cyclic peptides (romidepsin), benzamides (entinostat), and short-chain fatty acids (butyrate, valproic acid). Several HDAC inhibitors are FDA-approved for cancer therapy.

Do HDACs only act on histones?

No. HDACs deacetylate numerous non-histone proteins, including transcription factors (p53, NF-κB), cytoskeletal proteins (α-tubulin), chaperones (Hsp90), and metabolic enzymes. The number of known non-histone substrates exceeds that of histone substrates.

Key Takeaways

HDACs remove acetyl groups from lysine residues on histones, promoting chromatin compaction and transcriptional repression.
The 18 human HDACs are classified into four groups: zinc-dependent Classes I, II, and IV, and NAD⁺-dependent Class III sirtuins.
HDACs function within large multiprotein co-repressor complexes that are recruited to specific genomic loci by transcription factors.
HDAC activity is regulated by phosphorylation, ubiquitination, and interaction with co-repressor complexes.
HDACs deacetylate hundreds of non-histone proteins, expanding their functional repertoire beyond chromatin regulation.
HDAC inhibitors are approved for cancer therapy but have significant side effects due to their broad activity.
HDAC inhibition does not uniformly activate gene expression; the transcriptional effects are context-dependent and cell-type specific.