Biochemistry 🧪 Flashcards
What are porphyrins?
are cyclic compounds synthesized in the human body
What do porphyrins form?
metalloporphyrins (Haem). “After conjugation with metals”
What is the result of the conjugation of heme with proteins?
Haemoproteins:
1) Haemoglobin of RBCs.
2) Myoglobin of muscle.
3) Cytochromes: Respiratory enzymes in the electron transport chain.
What contains FE-porphyrins as prosthetic “synthetic” groups?
Haemoproteins:
1) Haemoglobin of RBCs.
2) Myoglobin of muscle.
3) Cytochromes: Respiratory enzymes in the electron transport chain.
What is porphyrins chemistry?
- They are complex structures consisting of 4 pyrrole rings, united by “methylene” bridges.
- The nitrogen of 4 pyrrole rings can form complex with metallic ions such as Fe++
What are the sites of synthesis of heme?
- Porphyrins are synthesized partly in the mitochondria and partly in the cytosol of most body tissues except mature red blood cells (due to lack of mitochondria) mainly in:
Bone marrow (80%). “Site of formation of RBCs”
Liver (15%). “Many mitochondria, hemoproteins that detoxify”
What are the steps of the synthesis of heme?
Step 1: Synthesis of δ-Amino Levulinic Acid (δ-ALA) (Intramitochondrial)
Step 2: Formation of Porphobilinogen (PBG) (Cytosolic)
Step 3 & 4: Formation of Uroporphyrinogen (Cytosolic)
Step 5: Formation of Coproporphyrinogen: (Cytosolic)
Step 6-8: Formation of haem: (Intramitochondrial)
What are the events that happened in step one of heme synthesis?
Glycine and succinyl CoA condense to form ALA in a reaction catalyzed by ALA synthase.
What is the rate-controlling step in hepatic porphyrin biosynthesis?
Step 1
What are the events that happen in step two of heme synthesis?
Two molecules of ALA condense to form porphobilinogen (PBG) “first payroll ring compound” by ALA dehydratase. “Removal of h2o”
What is ala dehydratase sensitive to?
ALA dehydratase is extremely sensitive to inhibition by heavy metal ions.
What does lead poisoning lead to?
leads to the accumulation of ALA and anemia.
What are the events that happen in step three of haem synthesis?
Four molecules of porphobilinogens (PBG) are condensed together by PBG deaminase, producing the linear tetrapyrrole, hydroxymethylbilane.
What are the events that happen in step four of haem synthesis?
- Ring closure and isomerization of hydroxymethylbilane produce uroporphyrinogen III.
- This reaction is catalyzed by the enzyme uroporphyrinogen III synthase.
What are the events that happen in step five of heme synthesis?
Uroporphyrinogens undergo decarboxylation of their acetate groups, catalyzed by the enzyme uroporphyrinogen decarboxylase, generating coproporphyrinogens.
“Uro—->copro”
What are the events that take place in steps from No. 6 to number 8 in heme synthesis?
The 2 side chains of coproporphyrinogen are decarboxylated generating protoporphyrinogen IX. “Copro—>proto”
Proto-porphyrinogen IX is oxidized to proto-porphyrin IX.
The introduction of iron (as Fe+2) into proto-porphyrin IX occurs spontaneously, but the rate is enhanced by the ferrochelatase enzyme, which is inhibited by lead.
How is heme regulation Done?
The key regulatory enzyme is δ-ALA synthase (in mitochondria).
What are the regulatory influences and effects of inhibitors in heme synthesis
1) Effect of O2: haem synthesis is stimulated by low O2 tension (e.g. living at high altitudes). “Negative feedback”
2) Steroids: Stimulate δ-ALA synthetase.
3) Iron: Stimulate δ-ALA synthetase.
4) Lead: It is known to produce profound abnormalities in porphyrin metabolism. It inhibits δ-ALA synthetase, δ-ALA dehydratase “most sensitive” and haem synthetase.
“Stimulate—>ISO”
What is the definition of Porphyrias?
❖ Porphyrias are the metabolic disorders of heme synthesis, characterized by accumulation and increased excretion of porphyrins or their precursors.
What causes porphyrias?
❖ Porphyrias may be due to deficiency of one or more of heme biosynthesis enzymes → ↓ heme production → ↑ porphyrins.
❖ E.g.: uroporphyrin synthetase - uroporphyrin decarboxylase – coproporphyrin oxidase.
what are the types of porphyrias?
❖ Porphyrias are either congenital or acquired.
❖ Acquired porphyria may be due to intake of:
1) Sulfonamides (antibacterial).
2) Sulfonylureas (antidiabetic).
3) Barbiturates (anticonvulsant).
4) Antifungals e.g. griseofulvin.
❖ The most common acquired form of porphyria is due to lead poisoning. “Non-drug induced”
“Sulf sulf barb fungal”
What are the manifestations of porphyrias?
1) Abdominal pain.
2) Neuropsychiatric symptoms.
3) Photosensitivity: There are skin itches and burns (pruritis) when exposed to visible light
4) Porphyrinuria: Porphyrins are excreted in urine → dark urine.
What causes photosensitivity in porphyrias?
Due to the presence of increased amounts of uro-, copro-, protoporphyrins in tissues with strong absorption of light at 400 nm wavelength, associated with the release of energy and free radicals → tissue damage.
What is the definition of hemoglobin?
Tetrameric protein molecule found exclusively in the cytoplasm of Red blood cells (RBCS).
What is the main function of hemoglobin?
Is to transport oxygen from lungs to the tissues & carbon dioxide and H protons from tissues to lung “To be exerted, so it maintains acid base balance”
What is the chemical nature of hemoglobin?
Conjugated protein: formed of protein part: globin & non- protein part: heme group “prosthetic group”
Hemoprotein: contains heme as a tightly bound prosthetic group
Chromoprotein: has a red colour in vertebrates as it contains a pigmented prosthetic heme group (or cofactor), which is the iron containing molecule that makes oxygenated blood appear red
What is the chemical structure of hemoglobin?
- Hb is a tetramer consisting of 2 parts:
Heme: 4 heme molecules
Globin: 4 polypeptide chains
What is the chemical structure of heme?
Heme is a complex of tetrapyrrol ring called protoporphyrin IX & ferrous iron (Fe2+)
The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring.
The Heme Fe2+ can form two additional bonds, one on each side of the porphyrin ring:
✓ One of these positions is coordinated to the side chain of a histidine amino acid of the globin molecule, Whereas the other position is available to bind oxygen
What is globin?
Globin is a protein rich in histidine amino acid, with a quaternary structure (formed of 4 polypeptide chains)
What does each chain of globin contain?
Each chain (subunit) has stretches of α-helical structure and a hydrophobic heme- binding pocket.
What are the types of globin chains?
There are four types of globin chains: α, β, γ and δ (According to sequence of amino acids in the primary structure of each chain)
What is the type of bonds between the chains of hemoglobin and what is the number of amino acids in each chain?
non-covalent bonds:
✓ α -chains contain 141 amino acids.
✓ β -chains contain 146 amino acids.
What are the types of hemoglobin classified according to?
the primary structure of the globin polypeptide chains.
What are the types of hemoglobin?
Hb A or HbA1 (Adult Hb)
Hemoglobin A2 (HbA2)
Fetal hemoglobin (Hb F)
Hemoglobin A1c (HBA1c) “doesn’t affect the function”
What does HBA1 represent and what is composed of?
Is the major form of normal Hb in adults represents about 90-95% of total Hb. “ 6 months after birth”
It is composed of 2 α and 2 β chains.
What does HBA2 represent and what is it composed of?
Minor adult Hb, comprised 2-3% of normal adult Hb.
Composed of 2 α and 2 δ chains
What does HBF represent what is it composed of?
Is the main Hb during fetal life and in newborns then disappear gradually where it is replaced by Hb A shortly after birth.
It is composed of 2α and 2 γ chains.
What does HBA1C represent and what is it composed of?
- Normally, it is present in conc. of 3 - 5 % of total Hb. However, in patients with D.M. it may be increased to as much as 6 -15 % of total Hb.
- Formed spontaneously by nonenzymatic reaction of hemoglobin A with Glucose (HBA1c has a glucose residues attached to β-globin chains in RBCs hemoglobin by non-enzymatic reaction).
What does a greater affinity to O2 HBA or HBF?
- Hb F has greater affinity for O2 than HbA so ensure O2 transfer from maternal circulation to fetus RBCs through placenta. “May cause problems” “less than 1% in adults”
What does the extent of glycosylation depend on in HBA1C?
the plasma conc. of glucose.
What is HBA1c used for?
HbA1c could be used as a monitor for the control of the blood glucose level during the last 2 months for diabetic patients. “ HB lifetime is two months “
What are hemoglobinopathies?
- They are members of a family of genetic disorders caused by genetic mutations in the genes producing globin protein of hemoglobin
What are hemoglobinopathies characterized by?
- Production of a structurally abnormal hemoglobin molecule (Qualitative hemoglobinopathies): eg: HbS & HbM.
- Synthesis of insufficient quantities of normal hemoglobin (Quantitative hemoglobinopathies): eg: Thalasaemias.
What is sickle cell anemia and what causes it (HB S disease)?
- Genetic disorder caused by mutation in 6th codon of the β-globin “In globin of hemoglobin” gene where glutamic acid (GAG) is replaced by valine (GTG).
What is a pathogenesis of sickle cell anemia?
- Valine residues aggregate together by hydrophobic interactions leading to precipitation of abnormal Hb within RBCs (Hb S).
What is the result of sickle cell anemia?
- RBCs which are sickle-shaped lead to fragility of their walls and high rate of hemolysis
- Such sickled cells frequently block flow of blood in narrow capillaries and block blood supply to tissue (tissue anoxia) causing pain and cell death.
What is the definition of thalassemias?
A group of genetic diseases in which synthesis of either α- or β- globin chain is defective.
What are the causes of thalassemias?
This due to defect or absence of one or more of genes responsible for synthesis of α or β globin chains of Hb molecule leading to malformed RBC.
“Not just a mutation in the 6th codon of the gene responsible for the synthesis of beta globin chain”
What are the types of thalassemias?
- α-thalassemia: in which synthesis of α globin chain is defective or absent
- β -thalassemia: When synthesis of β globin chain is decreased or absent.“May cause death”
What are the sites of hemoglobin degradation and when does it happen?
- In reticuloendothelial system: liver, spleen &bone marrow
- After approximately 120 days in the circulation: RBCs taken up by RES especially spleen and liver where HB become degraded.
What are the steps of hemoglobin degradation?
1) Formation of Bilirubin
2) Uptake of bilirubin by liver
3) Conjugation of bilirubin (Formation of bilirubin diglucuronide)
4) Excretion of bilirubin into bile
What happens when RBCs end their lifespan?
- The globin is degraded to amino acids (which are reutilized in the body)
- Heme releases iron (which is returned to the body’s iron stores), and the tetrapyrrole component is converted to bilirubin, which is mainly excreted into the bowel via the bile.
Step one: formation of bilirubin
- Heme under the effect of heme oxygenase “in macrophages” in presence of NADPH+H become converted to Biliverdin, and iron become oxidized into ferric and released.
- Biliverdin is then reduced via biliverdin reductase enzyme and become converted into bilirubin.
“Heme to bilivirdin”
“Ferrous to ferric”
Step two: uptake of bilirubin by liver
- Bilirubin is hydrophobic, so, it is transported in blood by albumin [ (unconjugated) (Indirect bilirubin (Hemobilirubin)]
- Then bilirubin uptake by the hepatocytes takes place where it dissociated from albumin.
Step three: conjugation of bilirubin (formation of bilirubin diglucuronide)
- Conjugation of bilirubin to 2 molecules of glucuronic acids occurs in hepatocytes to increase its solubility.
✓ This catalyzed by bilirubin UDP glucuronyltransferase enzyme.
- The product is bilirubin diglucuronide (conjugated bilirubin) (direct bilirubin) (cholebilirubin).
What is other name for unconjugated bilirubin what is the other name for conjugated bilirubin?
Unconjugated bilirubin is referred to as indirect bilirubin, while conjugated bilirubin is referred to as direct bilirubin
Step 4: excretion of bilirubin in to bile
- Conjugated Bilirubin passes with the bile to the large intestine where glucuronic acid is removed leaving bilirubin. “No need anymore”
- It is converted into stercobilinogen and urobilinogen by intestinal bacteria.
✓ Then stercobilinogen is then oxidized to stercobilin which give feces it characteristic brown color
- Some urobilinogen is reabsorbed into blood via portal circulation to the liver and then re-excreted into bile (enterohepatic urobilinogen cycle).
- The remaining urobilinogen is transported via blood to the kidney
✓ Converted to yellow urobilin and excreted through urine.
✓ This gives urine its characteristic color
What does the increase in the level of blood bilirubin (hyperbilirubinemia) indicate?
Increase in the level of the blood bilirubin (hyperbilirubinemia) is a case of Jaundice (yellow discoloration of skin, conjunctiva and mucous membrane)
What is the composition of plasma?
Water: 90 %
Inorganic substances: 0.9%
- Cations: Na+ / K+
- Anions: Cl / HCO3 / PO4 / SO4
Organic substances: 9.1 %
- Plasma proteins: (6-8 gm%) (6-8GM/DL)
- Lipids
- Others
What is the level of total proteins in plasma?
about 7-7.5 g/dl. “dl=100ml”
What is the composition of plasma proteins?
Plasma proteins include not only simple proteins but also conjugated proteins as glycoproteins and lipoproteins.
What are the types of plasma proteins?
Plasma proteins can be classified into 3 main groups: albumin, globulins (α1, α2, β and y ) and fibrinogen.
Where are plasma proteins synthesized?
Most plasma proteins are synthesized in the liver. However, Y globulins “antibodies” are synthesized by plasma cells and B cells of lymphoid tissues.
What is the definition of albumin and what is its levels in blood?
- Is the major human plasma protein (60% of total plasma protein).
- 3.5-5.5 g/dl
What are the functions of albumin?
“Osmotic pressure - carrier - treatment of hemorrhage and burns”
- Responsible for 70 to 80% of osmotic pressure of human plasma.
- Helps in transport of several substances e.g., FFA, unconjugated bilirubin, Ca++ and steroid hormones”also carried by globulins”.
- Certain drugs also bind to albumin, e.g., sulphonamides, aspirin, penicillin and are transported to target tissue.
- Preparations of human albumin have been widely used in treatment of hemorrhagic shock and burns.
What are alpha-1 globulins? “Markers”
α1-acid glycoprotein (orosomucoid)
α1-antitrypsin (α- AT)
What is the function of alpha 1 - acid Glycoproteins (Orosomucoid)?
A reliable indicator of acute inflammation. “Its increase”
What is the function of alpha 1 - antitrypsin?
It is the principal protease inhibitor (Pi) of human plasma: It inhibits trypsin, elastase and other proteases by forming complexes with them.
- It is deficient in emphysema.
What are alpha-2 globulins? “Carriers”
Haptoglobin
What is the function of haptoglobulin?
- Bind free Hb and minimizes urinary loss of Hb.
- After binding, Hp-Hb complex circulates in the blood,
which cannot pass through glomerular filter and ultimately the complex is destroyed by RE cells.
What are beta globulins?
β- Lipoproteins (LDL) “bad”
Transferrin “carrier”
C-reactive Protein
What is the function of transferrin?
transport of Fe between intestine and site of synthesis of Hb and other Fe containing proteins.
What is the function of C-reactive proteins?
- It precipitates with group C polysaccharide of pneumococci, in the presence of Ca, hence the name.
- It can bind to T-lymphocytes and can activate complement.
- Used as a marker of tissue injury & inflammation.
What are gamma globulins?
These are immunoglobulins having antibody activity.
What is fibrinogen?
- also called clotting factor I,
- as it takes part in coagulation of blood.
What are the functions of plasma proteins?
Nutritive Fluid exchange (colloid osmotic pressure) Buffering action Binding and transport function Blood coagulation and fibrinolysis Immunological function Enzymes Carriage of CO2
What is the nutritive function of plasma proteins?
contribute amino acids for tissue protein synthesis.
How do plasma proteins affect fluid exchange?
Regulation of blood volume and tissue fluid formation: plasma proteins exert osmotic pressure across the capillary wall which tend to pull water into the blood.
What is the buffering action of plasma proteins?
The serum proteins, like other proteins, are amphoteric, they have free acidic (R-COOH) and basic (R-NH2) end so can act as weak acid or base and thus can combine with acids or bases.
What do plasma proteins transport?
✓ Albumin → binds various ligands (thyroxine, steroids, a a, vitamins and FA).
✓ Haptoglobin→binds extra corpuscular HB
✓ Apolipoproteins → Lipoproteins
✓ Thyroid-binding globulin → binds T3 & T4
✓ Transferrin → transports iron.
✓ Ceruplasmin → copper
✓ Steroid hormone – binding globulin → steroid hormone
✓ Transcobalamin→vit B12 “cobolamin”
What is the Immunological function of plasma proteins?
Y-globulins protect the body against microbial infections.
What is the enzymatic function of plasma proteins?
Enzymes are proteins.
How do plasma proteins Carry co2?
Co2 combines with the amino group of the plasma proteins and is carried as carbamino compounds.
What is the most common method of analyzing plasma proteins?
Electrophoresis
Mention a method used to analyze plasma proteins.
Serum protein electrophoresis: By cellulose acetate electrophoresis, the serum can be separated into a number of fractions (albumin, α1, α2, β and γ globulins).
What does deficiency of albumin in Cellulose acetate electrophoresis indicates?
Nephrotic syndrome or cirrhosis of liver
What are acute phase proteins (or reactants)?
Levels of certain proteins in plasma increase during acute inflammatory states or secondary to certain types of tissue damage. These proteins are called Acute phase proteins or reactants.
What are examples of acute phase proteins?
C-reactive protein (CRP)
Haptoglobin (Hp)
α1 antitrypsin
α1 acid glycoprotein (orosomucoid) and fibrinogen
“All studied except LDL and transferrin”
What are the causes of hypoproteinemia?
Hemodilution: overload from IV infusion.
Hypoalbuminemia:
Hypogammaglobulinemia
What are the causes of hypoalbuminemia?
Loss from the body:
- Renal: Loss of albumin in urine in nephrotic syndrome. - - GI Tract: Protein losing enteropathy.
- Skin: Burns and other exudative skin lesions.
Decreased synthesis of albumin:
- Severe liver diseases: Chronic hepatitis, cirrhosis liver.
- Genetic deficiency: Analbuminaemic “rare, the gene responsible”
Miscellaneous: Pregnancy, chronic illness
What are the causes of hypogammaglobinemia?
✓ Protein loss: Same as above.
✓ Decreased synthesis:
o Primary: Genetic deficiency “very rare”
o Secondary” suppressing the immunity” : Certain toxins/and drugs: uremia, cytotoxic therapy, corticosteroid therapy, AIDS
✓ Miscellaneous: Pregnancy.
What are the effects of hypoproteinemia?
- Swelling of face and other parts of the body. “Due to loss of oncotic pressure”
- Edema L.L. “Due to loss of oncotic pressure”
- Loss of muscle mass “Deficiency of proteins”
- Infections “Decreased immunity”
- Fatigue “Decreased immunity”
- Dry brittle hair
- Lack of growth in children.
