Carbonic anhydrase

Carbonate Dehydratase
Ribbon diagram of human carbonic anhydrase II, with zinc ion visible in the center
Identifiers
EC no.	4.2.1.1
CAS no.	9001-03-0
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / QuickGO
Search PMC articles PubMed articles NCBI proteins

Eukaryotic-type carbonic anhydrase
Identifiers
Symbol	Carb_anhydrase
Pfam	PF00194
InterPro	IPR001148
PROSITE	PDOC00146
SCOP2	1can / SCOPe / SUPFAM
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary PDB 1kopB:33-252 1rj6B:22-278 1rj5B:22-278 1jczA:32-289 1jd0A:32-289 1g6vA:5-253 1v9eB:5-258 1v9iC:5-258 1cak :5-259 1i9qA:5-259 1yo0A:5-259 1zsa :5-259 1cnc :5-258 1bnw :5-258 1fqlA:5-259 1cim :5-258 1h9n :5-259 1g0eA:5-259 1g46A:5-259 1xevC:5-259 1ydb :5-258 1fsrB:5-259 1cve :5-258 1zsb :5-259 1fr7A:5-259 1bv3A:5-259 1cnj :5-259 1fsqB:5-259 1cam :5-259 1lzvA:5-259 1if4A:5-259 1t9nA:5-259 1caj :5-259 1f2wA:5-259 2cba :5-259 1bn1 :5-259 1cai :5-259 1i9lA:5-259 1g3zA:5-259 1kwqA:5-259 2cbe :5-259 1raz :5-259 1xpzA:5-259 2abeA:5-259 1ugg :5-259 1rzd :5-259 1te3X:5-259 1dca :5-258 1g52A:5-259 1yda :5-258 1can :5-259 12ca :5-258 1cvh :5-258 1ca3 :5-258 1fqnA:5-259 9ca2 :5-258 1if5A:5-259 1h9q :5-258 5ca2 :5-258 1heb :5-258 1mooA:5-259 4ca2 :5-258 1g45A:5-259 1mua :5-258 1lg6A:5-259 1ca2 :5-258 1i91A:5-259 1hva :5-258 1ugc :5-259 1xegA:5-259 1hca :5-258 1avn :5-259 1am6 :5-259 1fsnB:5-259 1g1dA:5-259 2cbc :5-259 1fqrA:5-259 1cng :5-259 1g53A:5-259 1ccu :5-258 1teqX:5-259 1caz :5-259 1ccs :5-258 1oq5A:5-259 1bnq :5-258 1bnu :5-258 1cil :5-258 1rze :5-259 8ca2 :5-258 1ray :5-259 1i8zA:5-259 2cbd :5-259 1eouA:5-259 1okl :5-258 1ze8A:5-259 1g0fA:5-259 4cac :5-258 1cay :5-259 1tg9A:5-259 1hec :5-258 1yo2A:5-259 6ca2 :5-258 1okn :5-259 1cvc :5-258 1ttmA:5-259 1cnb :5-258 1th9A:5-259 1lg5A:5-259 1zsc :5-259 2cbb :5-259 1bcd :5-259 1bnv :5-259 1h4n :5-259 1bic :5-259 1g4jA:5-259 1g54A:5-259 1cal :5-259 1tb0X:5-259 1ugd :5-259 1i9mA:5-259 1cah :5-259 1a42 :5-258 1thkA:5-259 1kwrA:5-259 1g4oA:5-259 3ca2 :5-258 1ugf :5-259 1cvb :5-258 1rzb :5-259 1lgdA:5-259 1bn4 :5-259 1fqmA:5-259 1cao :5-259 1cni :5-259 1cnx :5-258 2ca2 :5-258 1bnm :5-259 1yo1A:5-259 1rzc :5-259 1uge :5-259 7ca2 :5-258 1uga :5-259 1cra :5-259 2ax2A:5-259 1bnn :5-259 2h4n :5-259 1dcb :5-258 1cny :5-258 1i9oA:5-259 1if8A:5-259 1ydc :5-258 1bnt :5-258 5cac :5-258 1rza :5-259 1cnk :5-259 1cnw :5-258 1tbtX:5-259 1i9pA:5-259 1hea :5-258 1if9A:5-259 1i90A:5-259 1cvf :5-258 1fr4A:5-259 1cvd :5-258 1if7A:5-259 1cin :5-258 1cva :5-258 1i9nA:5-259 1ydd :5-258 1bn3 :5-259 1teuX:5-259 1hed :5-258 1tg3A:5-259 1cnh :5-259 1okm :5-259 1if6A:5-259 1cct :5-258 1g48A:5-259 1xq0A:5-259 1q4iA:5-259 1crm :6-260 1hcb :6-260 1huh :6-260 1hug :6-260 1j9wA:6-260 1czm :6-260 1jv0B:6-260 1bzm :6-260 1azm :6-260 2cab :6-260 1z97A:5-259 1z93A:5-259 1fljA:5-259 1dmxA:55-290 1urt :55-290 1dmyA:55-290 1keqB:53-290 2znc :22-278 3znc :22-278 1zncB:23-284

The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons (or vice-versa), a reversible reaction that occurs rather slowly in the absence of a catalyst. The active site of most carbonic anhydrases contains a zinc ion; they are therefore classified as metalloenzymes.

One of the functions of the enzyme in animals is to interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues, and to help transport carbon dioxide out of tissues.

Reaction

The reaction catalyzed by carbonic anhydrase is:

${\rm {CO_{2}+H_{2}O\rightarrow ^{Carbonic\ anhydrase}HCO_{3}^{-}+H^{+}}}$(in tissues - high CO₂ concentration)

The reaction rate of carbonic anhydrase is one of the fastest of all enzymes, and its rate is typically limited by the diffusion rate of its substrates. Typical catalytic rates of the different forms of this enzyme ranging between 10⁴ and 10⁶ reactions per second.

The reverse reaction is also relatively slow (kinetics in the 15-second range) but is sped up by carbonic anhydrase. Carbonic anhydrase found in saliva helps create CO₂ when a carbonated drink is taken into the mouth.

An anhydrase is defined as an enzyme that catalyzes the removal of a water molecule from a compound, and so it is this "reverse" reaction that gives carbonic anhydrase its name, because it removes a water molecule from carbonic acid.

${\rm {HCO_{3}^{-}+H^{+}\rightarrow H_{2}CO_{3}\rightarrow CO_{2}+H_{2}O}}$ (in lungs and nephrons of the kidney - low CO₂ concentration, in plant cells)

Mechanism

Close-up rendering of active site of human carbonic anhydrase II, showing three histidine residues (in pink) and a hydroxide group (red and white) coordinating the zinc ion (purple). From PDB: 1CA2​.

A zinc prosthetic group in the enzyme is coordinated in three positions by histidine side-chains. The fourth coordination position is occupied by water. This causes polarisation of the hydrogen-oxygen bond, making the oxygen slightly more negative, thereby weakening the bond.

A fourth histidine is placed close to the substrate of water and accepts a proton, in an example of general acid-general base catalysis. This leaves a hydroxide attached to the zinc.

The active site also contains specificity pocket for carbon dioxide, bringing it close to the hydroxide group. This allows the electron rich hydroxide to attack the carbon dioxide, forming bicarbonate.

CA families

Ribbon diagram of human carbonic anhydrase II. Active site zinc ion visible at center. From PDB: 1CA2​.

There are at least five distinct CA families (α, β, γ, δ and ε). These families have no significant amino acid sequence similarity and in most cases are thought to be an example of convergent evolution. The α-CAs are found in humans.

α-CA

The CA enzymes found in mammals are divided into four broad subgroups, which, in turn consist of several isoforms:

• the cytosolic CAs (CA-I, CA-II, CA-III, CA-VII and CA XIII) (CA1, CA2, CA3, CA7, CA13)
• mitochondrial CAs (CA-VA and CA-VB) (CA5A, CA5B)
• secreted CAs (CA-VI) (CA6)
• membrane-associated CAs (CA-IV, CA-IX, CA-XII, CA-XIV and CA-XV) (CA4, CA9, CA12, CA14)

There are three additional "catalytic" CA isoforms (CA-VIII, CA-X, and CA-XI) (CA8, CA10, CA11) whose functions remain unclear.

Comparison of mammalian carbonic anhydrases

Isoform	Gene	Molecular mass	Location (cell)	Location (tissue)	Specific activity of human enzymes (except for mouse CA XV) (s−1)	Sensitivity to sulfonamides (acetazolamide in this table) KI (nM)
CA-I	CA1	29 kDa	cytosol	red blood cell and GI tract	2.0 × 105	250
CA-II	CA2	29 kDa	cytosol	almost ubiquitous	1.4 × 106	12
CA-III	CA3	29 kDa	cytosol	8% of soluble protein in Type I muscle	1.3 × 104	240000
CA-IV	CA4	35 kDa	extracellular GPI-linked	GI tract, kidney, endothelium	1.1 × 106	74
CA-VA	CA5A	34.7 kDa (predicted)	mitochondria	liver	2.9 × 105	63
CA-VB	CA5B	36.4 kDa (predicted)	mitochondria	widely distributed	9.5 × 105	54
CA-VI	CA6	39-42 kDa	secretory	saliva and milk	3.4 × 105	11
CA-VII	CA7	29 kDa	cytosol	widely distributed	9.5 × 105	2.5
CA-IX	CA9	54, 58 kDa	cell membrane-associated	normal GI tract, several cancers	1.1 × 106	16
CA-XII	CA12	44 kDa	extracellularily located active site	kidney, certain cancers	4.2 × 105	5.7
CA-XIII	CA13	29 kDa	cytosol	widely distributed	1.5 × 105	16
CA-XIV	CA14	54 kDa	extracellularily located active site	kidney, heart, skeletal muscle, brain	3.1 × 105	41
CA-XV	CA15	34-36 kDa	extracellular GPI-linked	kidney, not expressed in human tissues	4.7 × 105	72

β-CA

Most prokaryotic and plant chloroplast CAs belong to the beta family. Two signature patterns for this family have been identified:

• C-[SA]-D-S-R-[LIVM]-x-[AP]
• [EQ]-[YF]-A-[LIVM]-x(2)-[LIVM]-x(4)-[LIVMF](3)-x-G-H-x(2)-C-G

γ-CA

The gamma class of CAs come from methane-producing bacteria that grow in hot springs.

δ-CA

The delta class of CAs has been described in diatoms. The distinction of this class of CA has recently come into question, however.

ε-CA

The epsilon class of CAs occurs exclusively in bacteria in a few chemolithotrophs and marine cyanobacteria that contain cso-carboxysomes. Recent 3-dimensional analyses suggest that ε-CA bears some structural resemblance to β-CA, particularly near the metal ion site. Thus, the two forms may be distantly related, even though the underlying amino acid sequence has since diverged considerably.

Pharmacological agents affecting CA

See Carbonic anhydrase inhibitors

Structure and function of carbonic anhydrase

Several forms of carbonic anhydrase occur in nature. In the best-studied α-carbonic anhydrase form present in animals, the zinc ion is coordinated by the imidazole rings of 3 histidine residues, His94, His96, and His119.

Carbonic Anhydrase Enzymes produce hydrogen ions that are located in the mitochondria of all living things and are utilized as: 1. Fuel of the ion pump that maintains the integrity of the cell wall membrane. 2. Fuel of all metabolic activities- 1/2 for cell division and 1/2 for all other activities.

H⁺ + ADP (adenosine diphosphate) → ATP (adenosine triphosphate)

3. Neutralizes all reactive radicals including reactive oxygen, illustrated in the following steps:

glucose + Oxygen → H₂0 + CO₂ + (O₂)^-2 (reactive oxygen—1 to 5% are produced)

H₂0 + C0₂ ⇌ H⁺ + HCO₃^- (reversible reaction acted upon by carbonic anhydrase enzymes)

Hydrogen ions produced by Carbonic Anhydrase Enzymes

H⁺ + (O₂)^-2 → O₂H^- (Superoxide)

H⁺ + (O₂H)^- → H₂O₂ (Hydrogen peroxide)

H₂O₂ → OH^- (Hydroxyl)

2H⁺ + 2OH^- → 2H₂O (water final product)

4. Hydrogen ions produced are also utilized to prevent misfolding of proteins. They are used to form hydrogen bonds hence making them more stable.

Lack of Carbonic anhydrase enzymes results to reduction of hydrogen ions produced leading to increased levels of reactive radicals. These reactive radicals binds with almost all cellular and non-cellular elements of all living things leading to cellular death. They are implicated to almost all diseases of mankind,including cancer and aging.

Reference: http://www.uspto.gov U.S. patent # 7858602(December 28, 2010) Rodriguez Therapeutic and Prophylactic uses of Cell Specific Carbonic Anhydrase Enzymes in Treating Aging Disorders due to Oxidative Stress and as Growth Factors of Stem Cells

One of the functions of the enzyme in animals is to interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues, and to help transport carbon dioxide out of tissues.

There exist at least 14 different isoforms in mammals. Plants contain a different form called β-carbonic anhydrase, which, from an evolutionary standpoint, is a distinct enzyme, but participates in the same reaction and also uses a zinc ion in its active site. In plants, carbonic anhydrase helps raise the concentration of CO₂ within the chloroplast in order to increase the carboxylation rate of the enzyme RuBisCO. This is the reaction that integrates CO₂ into organic carbon sugars during photosynthesis, and can use only the CO₂ form of carbon, not carbonic acid or bicarbonate.

In 2000, a cadmium-containing carbonic anhydrase was found to be expressed in marine diatoms during zinc limitation. In the open ocean, zinc is often in such low concentrations that it can limit the growth of phytoplankton like diatoms; thus, a carbonic anhydrase using a different metal ion would be beneficial in these environments. Before this discovery, cadmium had, in general, been thought of as a very toxic heavy metal without biological function. As of 2005, this peculiar carbonic anhydrase form hosts the only known beneficial cadmium-dependent biological reaction.

External links

• PDB Molecule of the Month Carbonic anhydrase

Further reading

• Lyall V, Alam RI, Phan DQ, Ereso GL, Phan TH, Malik SA, Montrose MH, Chu S, Heck GL, Feldman GM, DeSimone JA (2001). "Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction". Am. J. Physiol., Cell Physiol. 281 (3): C1005–13. PMID 11502578. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)