Biosynthesis of purine nucleotides (B. Pharm 2nd semester) biochemistry

BIOSYNTHESIS OF PURINE NUCLEOTIDES 

Biosynthesis- It is a multi - step, enzyme catalysed process where substrate are converted into more complex product in living organism.

v In biosynthesis simple compounds are modified and converted into other compounds / joined together to form macromolecules.

v This process often consist of metabolic pathways.

v The purine are built upon a pre – existing ribose 5-phosphate.

v Liver is the major site for purine nucleotide.

v Erythrocytes, polymorph nuclear, leukocytes and brain cannot produce purine.

Purine synthesis

 Purine synthesis is the process where purine nucleotides, such as adenosine and guanosine, are synthesized by de novo synthesis in the body.

• The purine nucleotides are essential for buildings of block for DNA and RNA.
• The synthesis of purine nucleotides occurs through a series of enzymatic reactions which convert the simpler precursors into final products, and including converting of amino acids and other metabolites into nucleotide bases.
• Purine synthesis are regulating process which are essential for maintaining of proper balance of nucleotides in the body and various types of biological processes like- DNA replication, cell division, and energy metabolism.

Where,

 N1 – form amino group Aspartate.

C2 & C8- Form Formate

N3 & N9- form amide group of Glutamine

C4, C5, & N7 – Glycine

C6- form HCO₃- (Bicarbonate)

 Aspartate, formate, glutamine, glycine, and bicarbonate act as building blocks which are used for purine synthesis.

 Steps involved in purine biosynthesis-

Purine (adenine & guanine ) are derived from inosine-5’-monophosphate.

The purine synthesis starts with IMP synthesis

b

BIOSYNTHESIS OF PYRIMIIDINE

It is the simplest biosynthesis than the purine biosynthesis and done by both salvage and de novo pathways.

Pyrimidines are one carbon-nitrogen ring bases.

They contain only a six-membered nitrogen-containing ring.

There are four pyrimidines as thymine ( found in only DNA), Uracil is (found only in RNA). Cytosine (found in both DNA and RNA) and orotic acid (intermediate metabolites).

• Uracil = 2,4-dioxy pyrimidine
• Thymine = 2,4-dioxy-5-methyl pyrimidine
• Cytosine = 2-oxy-4-amino pyrimidine
• 
  
  Orotic acid = 2,4-dioxy-6-carboxy pyrimidine

Similar to purine synthesis, pyrimidine synthesis also happens via both salvage and de novo pathways.

Pyrimidines  is also known as “orotate pathway” is usually defined as the formation of UMP (Uridine monophosphate) from carbamoyl phosphate (CP).

 Pyrimidine skeleton-

N1,C6, C5 AND C4 FORM Aspartate

N3- form glutamine

C2 – form HCO3-( bicarbonate)        

De Novo Synthesis of Pyrimidine Nucleotide -

Starting materials for pyrimidine neucleotide biosynthesis are included in bicarbonate, amine from glutamine, and phosphate from ATP to make carbamoyl-phosphate (which are similar to the reaction of the urea cycle).

Joining of carbamoyl phosphate to aspartic acid (forming carbamoyl aspartate) is catalyzed by the most important regulatory enzyme of the cycle, aspartate transcarbamoylase (also called aspartate carbamoyl transferase).

 

                            Figure: De Novo Synthesis of Pyrimidine Nucleotides

ATCase (Aspartate carbamoyl transferase) is regulated by three compounds.

v One of these (aspartate) is a substrate and it activates the enzyme by binding on catalytic site and favoring the enzyme’s R state.

v  The other two regulators bind to regulatory subunits of the enzyme and either inhibit s(CTP- cytidine tri-phosphate) or activate (ATP) the enzyme.

v The reaction product, carbamoyl aspartate, is transformed in two reactions-

                   i.          Orotic acid which is, combined with phosphoribosyl pyrophosphate (PRPP).

                 ii.          The product of that reaction, orotidyl monophosphate (OMP) is decarboxylated to form the first pyrimidine nucleotide, UMP.

               iii.          Conversion of UMP to UDP is catalyzed by nucleoside monophosphate kinases (NMPs) and UDP is converted to UTP by nucleoside di phosphokinase (NDPK).

               iv.          UDP (nucleoside diphosphates) is a branch point deoxy ribonucleoside diphosphates, catalyzed by ribonucleotide reductases.

                 v.          UTP is converted to CTP by CTP synthase.

Catabolism of purine nucleotide –

Purine catabolism =  purines are broken down through multiple steps due to uric acid by enzyme eg- xanthine oxidase enzyme.

Purines are  key components of cellular energy system. Example, ATP, NAD.

Purines and pyrimidines may be synthesised by De Novo or recycled by salvage pathway from normal catabolism process.

In the last the end product of complete catabolism of purine is uric acid formed.

 Catabolism of purine nucleotide is active purine cycles, which consist of Phosphorolysis and salvage to monophosphate of purine nucleoside. Deoxyribonucleosides and ribonucleotides.

Purine nucleoside cycles are capable for converting urine deoxyribonucleotides  into  ribonucleotide derivatives.

We can also say that purines catabolism is uric acid.

Urate oxidase is used in other animals to excrete the more soluble allantoin(produce from Uric acid) as the final product but in humans these enzymes are not presents.

Urate is final product. uric acid is mainly produced in a liver and discharge into the urine through the kidney in humans and animals.

Purine catabolism process-

Disorder of purine metabolism-

Hyperuricemia-

Causes-

Uric acid is formed from purines ( chemical compound) which are nitrogen containing substances. It’s a waste product which are formed during purines breakdown in body which are present in some food materials.

 Example- Red meats(rabbit, pig,goat, dear ), Sea foods , Organ meats beans , food and drinks with high fructose corn syrup, alcohol

Generally body discharge itself of uric acid during urination. Hyperuricemia occurs when body produce too much of uric acid and unable to discharge through the urine. 

Value- In female 2.4 to 6.0mg/ dL

         In male in 3.4 to 7.0mg/dL 

Symptoms – Intense pain (Signal about threat injury / disease)

                     Discoloration or redness (change in natural skin)

                     Stiffness (Inability to move)

Untreated hyperuricemia affected- Bones, tendons , joints, ligament

Other health disorder- kidney & liver disease, high blood pressure, diabetes, high blood pressure etc. 

GOUT –

Causes- it causes during urate crystals are accumulate in joint and causes the inflammation and intense pain (8-12hr) of gout attack.

Normally uric acid dissolved in blood and passes through kidney into urine. Once in a while body produced more uric acid and kidney excreted in little amount of uric acid.

So uric acid pile up forming sharp, needle like urate crystal in surrounding the joint or tissues which are causes pain, inflammation and swelling.

 Most affected part of the body, especially the big toes and knee’s. When the big toes are involved it is known as “Podagra”. Uric Acid can also be deposited in the kidneys and urinary tract causing “Kidney Stones”, if left untreated this can also lead to “Kidney Damage”.

 Urate crystal are formed when uric acid level are increased in blood.

Uric acid are formed during break down of purines which are naturally found in body and some examples are- red meat, organ meat, liver,

Sea foods –anchovies, sardines, mussels, scallops, trout and tuna.

 Alcoholic beverages – Beer,fruit sugar.

Risk factors- Diet, over weight, medical condition, Family history, age and sex etc.

Complications- Recurrent gout(more severe condition), advanced gout(untreated gout), kidney stones

Diagnosis- Joint fluid test, blood test, x- ray imaging, ultrasound, dual- energy computerized tomography (DECT) .