Amino Acid/Proteins & their metabolism [Part 1] (Viva)

Q.1 What is an α-amino acid?

Amino acid-containing an amino group and a carboxyl group on the α-carbon atom.

Q.2 What are glucogenic amino acids?

Those amino acids which give rise to the intermediates of carbohydrate metabolism are called glucogenic amino acids.

Q.3 Name the glucogenic amino acids?

Alanine, arginine, aspartic acid, cysteine, glutamic acid, glycine, histidine, proline, methionine, serine, leucine, threonine.

Q.4 What are ketogenic amino acids?

Amino acids which give rise to acetyl CoA during its course of metabolism are called ketogenic amino acids.

Q.5 Name some amino acids which are both glucogenic and ketogenic.

  1. Phenylalanine.
  2. Tryptophan.
  3. Tyrosine

Q.6 What are essential amino acids?

Essential amino acids are those amino acids that can not be synthesized by the body and hence have to be supplied in the diet.

Q.7 Name ketogenic amino acids?

 Leucine

Q.8 Name the essential amino acids?

Isoleucine, Leucine, Tryptophan, Threonine, Phenylalanine, Valine, Methionine, Lysine.

The essential amino acids can be remembered using the formula “MTTVILPL”.

Q.9 What are semi-essential amino acids?

Those amino acids which are partly synthesized by the body are called semi-essential amino acids,
e.g. arginine and Histidine.

Q.10 What are non-essential amino acids?

Those amino acids which are synthesized by the body.

Q.11 How are amino acids classified?

Amino acids are classified mainly into three groups depending on their reaction in solution as follows:

  • Neutral amino acids
  • Acidic amino acids
  • Basic amino acids

Q.12 What are neutral amino acids? Give examples.

Neutral amino acids form the largest group and they can be further classified as follows:

Aliphatic amino acids:
Having an aliphatic side chain and are called simple monoamino monocarboxylic acids,
e.g. glycine (gly), alanine (ala), valline (val), leucine (leu) and isoleucine (Ile).
Hydroxyamino acids:
Having an -OH group,
e.g. serine (ser), threonine (thr).
Aromatic amino acids:
Contain a benzene ring,
e.g. phenylalanine (phe), tyrosine (tyr).
Heterocyclic amino acids:
Containing heterocyclic group like indole ring or imidazole ring,
e.g. tryptophan (trp), histidine (his).
Sulphur containing amino acids:
Containing, Sulphur,
e.g. methionine (met), cysteine (cys) and cystine.
Imino acids:
They do not have a free - NH2 group but have a basic, pyrrolidone ring,
e.g. proline (pro), hydroxyproline (hyp).

 

Q.13 What is the relationship between cysteine and cystine?

Two molecules of cysteine are joined together by S–S–bond to form one molecule of cystine. One molecule of cysteine on oxidation gives 2 molecules of cysteine.

Q.14 What are acidic amino acids? Give examples.

 Acidic amino acids have two -COOH groups and one NH2 group and they are monoamino dicarboxylic acids, e.g. aspartic acid (asp) glutamic acid (glu).

Q.15 What are basic amino acids? Give examples.

Basic amino acids have one -COOH group and two –NH2 groups and they are diaminomonocarboxylic acids, e.g. arginine (arg), lysine (lys), and hydroxy-lysine (hyl).

Note: Histidine (his) can also be classified as basic amino acid due to the presence of an imidazole ring.

Q.16 Which is the simplest of the amino acids?

Glycine is the simplest of the amino acids.

Q.17 What is the chemical name of glycine?

Chemically glycine is called as amino acetic acid.

Q.18 Which amino acids do not have an asymmetric carbon and do not show optical activity?

Glycine is the only amino acid which does not have an asymmetric carbon and does not exhibit optical activity.

Q.19 Name the amino acids that do not occur in proteins. Give 5 examples.

There are compounds similar to the basic skeletal structure of amino acid but do not occur in proteins.

Examples:

  • β-alanine found in coenzyme A
  • α-amino butyrate—a neurotransmitter
  • Taurine—present in bile salt
  • Ornithine and citrulline—metabolites of urea cycle
  • DOPA—precursor of pigment melanin.

Q.20 Give examples of three amino acids that produce specific biologic compounds.

Tyrosine: produces
– Thyroid hormones,
– Catecholamines, and
– Melanin pigments

Glycine: Required for
– Heme synthesis
– Formation of creatine
– Formation of glutathione

Tryptophan: Necessary for
– Formation of serotonin
– Forms vitamin nicotinic acid.

Q.21 How does an amino acid behave in an acidic and basic media?

  • Every amino acid has at least two ionizable groups:
  • The -NH2 group and
  • The -COOH group.
  • In acidic medium: The -NH2 group behaves as a base and accepts a proton and becomes positively charged (cationic form).
  • In basic medium: The -COOH group acts as a proton donor and the amino acid becomes negatively charged (anionic form).

Q.22 What is isoelectric pH and what are zwitterion (or hybrid ion)?

At a specific pH, the amino acids carry both the charges in equal number and thus exist as dipolar ion or zwitterion. At this point, the net charge on the amino acids is zero, i.e. the positive and negative charges on the amino acids equalize. The pH at which the amino acid or protein is in zwitterion form is called isoelectric pH (pI).

Q.23 What is the isoelectric pH (pI) for albumin, hemoglobin, and casein?

Isoelectric pH (pI) of:

  • Albumin is 4.7
  • Hemoglobin is 6.7
  • Casein is 4.6.

Q.24 Name the sulfur-containing amino acids.

  1. Methionine.
  2. Cysteine.
  3. Cystine

Q.25 Which sulfur containing amino acid is an essential amino acid?

 Methionine

Q.26 Deficiency of essential amino acid causes what?

The deficiency of essential amino acids leads to negative nitrogen balance.

Q.27 What are the sources of amino acids in the blood?

  1. Tissue protein breakdown.
  2. Dietary proteins.
  3. Intracellular synthesis.

Q.28 What is the role of amino acids?

  1. As a building block of proteins.
  2. As precursors of:
  3. Hormones
  4. Purines
  5. Pyrimidines
  6. Porphyrins
  7. Certain vitamins such as pantothenic acid, folic acid, etc.

Q.29 What are the amino acids containing aromatic rings?

  1. Phenylalanine.
  2. Tyrosine.
  3. Tryptophan.

Q.30 Guanidino group is present in which amino acid?

Arginine.

Q.31 Imidazole group is present in which amino acid?

Histidine

Q.32 Indole group is present in which amino acid?

Tryptophan

Q.33 Name an imino acid.

Proline

Q.34 How amino acids are detected?

  1. Aromatic amino acids containing benzene ring by Xantoproteic reaction.
  2. Hydroxy aromatic amino acids by Millon’s reaction.
  3. Tryptophan by Hopkin’s Cole reaction.
  4. Arginine by Sagakuchi reaction.

Q.35 What is active methionine?

 S-adenosyl methionine is called the active methionine.

Q.36 What is the role of methionine?

As methyl group donar.

Q.37 Which amino acid gives rise to nicotinic acid during its metabolism?

 Tryptophan

Q.38 What are proteins?

 Proteins are polypeptides, formed by the linking of amino acids through peptide bonds.

Q.39 Give the classification of proteins.

 Proteins are classified into:

  1. Simple proteins.
  2. Conjugated proteins.

Q.40 What is the configuration of amino acids present in the naturally occurring proteins?

L-configuration

Q.41 What are the general tests for proteins?

  1. Biuret reaction.
  2. Ninhydrin test.

Q.42 Give one common reaction for all amino acids.

All amino acids on heating with a solution of ninhydrin in acetone to about 100 oC give a purple-blue colored compound (Ninhydrin reaction).

Q.43 Which amino acids do not give purple-blue color in ninhydrin reaction?

Proline and hydroxyproline are exceptions and they give a yellow color.

Q.44 What is biuret reaction?

Protein solution in water, two or three drops of dilute copper sulfate, and excess of NaOH (about one ml) produces a pink or purple-violet color.

Q.45 What is the mechanism of biuret reaction?

The color depends upon the presence of 2 or more peptide bonds. It is due to the coordination of cupric ions with the unshared electron pairs of peptide nitrogen and the oxygen of water to form the colored coordinate complex.

Note: Thus di-peptides and free amino acids do not give the biuret test.

Q.46 Name one amino acid which can give +ve biuret reaction.

Only Histidine can give a +ve biuret reaction.

Q.47 What is xanthoproteic reaction?

The aromatic amino acids like phenylalanine, tyrosine, and tryptophan present in protein molecule give yellow precipitate when heated with conc HNO3. The reaction is due to the nitration of the aromatic ring.

Note: Collagen and gelatin do not give a positive reaction.

Q.48 Give a specific color test for tyrosine in protein.

Millon’s test: It is a specific color test for tyrosine of protein. Proteins having tyrosine give a white precipitate with

Millon’s reagent (10% mercurous chloride in H2SO4) on heating. On addition of NaNO2, the precipitate turns pink-red.

 

Q.49 Mention a specific color test for arginine in protein.

Sakaguchi test: It is a specific color test for the arginine of protein. Sakaguchi reagent consists of alcoholic α-naphthol and a drop of sodium hypobromite. The guanidine group of arginine reacts to give the red color.

Q.50 Mention a specific color test for sulfur-containing amino acids.

Lead acetate test: It is specific for sulfur containing amino acids. The proteins having S-containing amino acids when boiled with strong alkali split out sulfur as sodium sulfide which reacts with lead acetate to give black precipitate of lead sulfide (Pbs).

Q.51 What is Biuret?

Biuret is a compound formed by heating urea. It contains two peptide linkages. 


Q.52 What are the methods by which proteins can be estimated?

  1. Buret method.
  2. Lowry method.
  3. Microkjaldehl method.
  4. Spectroscopy

Q.53 What is the factor used in the conversion of nitrogen to protein?

 6.25

Q.54 How does the factor 6.25 come?

The average nitrogen content of proteins is 16 gm%, i.e. 16 gm of nitrogen is present in 100 gm protein. 1 gm of nitrogen is present in 6.25 gm protein.

Q.55 Name two dipeptides found in our body.

The two dipeptides present in muscle tissues are:

  • Carnosine: β-alanine + histidine
  • Anserine: β-alanine + methyl-histidine

Q.56 Name tripeptide of immense biological importance.

  • Glutathione is a tripeptide consisting of three amino acids—Glutamic acid + Cysteine + and Glycine.
  • It functions in the body in oxidation reduction system.

Q.57 What are fibrous proteins? Give two examples.

When the axial ratio of length: Width of a protein molecule is more than 10, it is called a fibrous protein.

Examples:

  • α-keratin from hair
  • Collagen of connective tissues.

Q.58 What are globular proteins? Give two examples.

When the axial ratio of length: width of a protein molecule is less than 10, it is called a globular protein.

Examples:

  • Hemoglobin
  • Myoglobin

Q.59 What are chromoproteins? Give suitable examples.

Chromoproteins are conjugated proteins that contain a simple protein with a colored substance as the prosthetic group.

Examples:

  • All hemoproteins are chromoproteins that contain “heme” as a prosthetic group, e.g. hemoglobin, cytochromes, catalase, peroxidase.
  • Flavoproteins contain riboflavin, a yellow colored substance.
  • Visual purple (rhodopsin) contains protein ‘opsin’ + prosthetic group 11- cis retinal.

Q.60 What are phosphoproteins? Give two examples that have dietary importance.

Phosphoproteins are conjugated proteins containing phosphoric acid as the prosthetic group. The phosphoric acid is esterified through the -OH groups of serine and threonine.

Examples:

  • Caseinogen of milk
  • Vitellin of egg yolk

Q.61 What are metalloproteins? Give three examples.

Metalloproteins are conjugated proteins that contain a metal ion as their prosthetic groups.

Examples:

  • Carbonic anhydrase contains zinc
  • Ceruloplasmin contains copper
  • Ferritin contains Fe

Q.62 What is the essential difference between glycoproteins and mucoproteins?

  • Both are conjugated proteins and contain carbohydrate moiety as a prosthetic group.
  • They differ by carbohydrate content in that glycoproteins contain less than 4% of carbohydrates whereas mucoproteins contain more than 4% of carbohydrates.

Q.63 How do primary derived proteins differ from secondary derived proteins? Give examples of each group.

Primary derived proteins are synonymous with denatured proteins. In this, the primary structure is not disturbed and peptide bonds remain intact.

Examples: Proteans, metaproteins-acid/ alkaline, coagulated proteins.

Secondary derived proteins are formed by progressive hydrolysis of the peptide bonds. Protein is gradually hydrolyzed to produce smaller molecules.

Examples: Proteoses (albumoses), peptones, peptides.

Q.64 What are the various levels of protein structure?

  1. Primary structure.
  2. Secondary structure.
  3. Tertiary structure.
  4. Quaternary structure.

Q.65 What is the primary structure?

Primary structure indicates the sequence of amino acids linked through peptide bonds. It has an N-terminus and for the other end C-terminus.

Q.66 What is an a-helix?

The coiling of polypeptide chain into a right-hand helix.

Q.67 How will you determine the N-terminal amino acid of a protein?

  1. Sanger’s method.
  2. Edman’s method.

Q.68 Which reagent is used in Sanger’s method?

 2,4 dinitrofluorobenzene (2,4 DNFB).

Q.69 Which reagent is used in Edman’s method?

Phenyl isothiocyanate.

Q.70 Which method is superior, Sanger’s or Edman’s?

Edman’s method is superior over Sanger’s method because Edman’s method involves the removal of one amino acid at a time from the N-amino acid of a peptide, leaving the remaining peptide chain intact. The process can be repeated and the sequence of amino acid is obtained from the N-terminal end. Whereas in Sanger’s method only the N-terminal amino acid is identified because, after the removal of N-terminal amino acid with DNFB, the remaining peptide chain breaks into amino acid mixture.

Q.71 What is the secondary structure?

  • Secondary structure refers to the twisting of the peptide chain into a helical form. Hydrogen bonds are responsible for the secondary structure.
  • Hydrogen bonds in secondary structure may form either: – α-helix or – β-pleated sheet.

Q.72 Name different secondary structures of proteins.

  1. α-helix.
  2. β-pleated sheath.
  3. β-bend.
  4. Super secondary (motifs).
  5. Non-repetitive secondary structures.

Q.73 Which amino acid is conspicuously absent in α-helix?

Proline is never found in α-helix and is conspicuously absent.

Q.74 Which amino acids tend to destabilize α-helix?

More polar residues such as arginine, glutamic acid, and serine may repel and destabilize α-helix.

Q.75 Which protein tends to form triple helix?

Collagen is rich in proline and hydroxyproline hence it cannot form a-helix or βpleated sheet; it forms a triple helix. The triple helix is stabilized by both non-covalent as well as covalent bonds.

Q.76 What is tertiary structure? Which bonds are found in tertiary structure?

  • Tertiary structure arises due to the folding of the helical structure into globular, ellipsoidal or any other conformations.
  • The folding is brought about by:
    – Disulfide bonds (s-s)
    – Polar or/salt linkages between atoms with positive and negative charges.
    – Hydrophobic bonds, and
    – Van der Waal forces.

 

Q.77 What is a quarternary structure?

When proteins consist of 2 or more peptide chains held together by non-covalent interactions or by covalent cross-links, it is referred to as the quarternary structure.

Examples:

  • Hemoglobin: A tetramer having 4 polypeptide chains
  • Lactate dehydrogenase (LDH): Four polypeptide chains.

Q.78 What is meant by +ve ion precipitation?

The +ve ions most commonly used are those of heavy metals like Pb2+, Zn++, Ca++, Hg++, etc. The metals precipitate proteins at the pH alkaline to its isoelectric pH (pI). At this pH, proteins behave as anions and +ve metal ions combine with -COO group to give insoluble precipitate of metal proteinate.

Q.79 What is meant by – ve ions precipitation?

Negative ions like tungstic acid, trichloroacetic acid, picric acid, tannic acid, etc. combine with proteins when the pH of the medium is on the acidic side of its isoelectric pH (pI). In acidic pH, proteins exist as Pr+ and forms precipitate with -ve ions.

Q.80 What is electrophoresis?

Migration of charged molecules under the influence of electricity is called electrophoresis.

Q.81 What are the fractions obtained in paper electrophoresis of serum?

Albumin being lighter moves fast. It is followed by α1 globulin, α2 globulin, βglobulin and last γ-globulin near the origin.

Q.82 What is RF value?

The ratio of the distance moved by a compound/amino acid in paper chromatography to the distance moved by the solvent front is known as Rf value.

Q.83 Name six important plasma proteins.

  • Six plasma proteins are:
  • Albumin, α1-globulin, α2-globulin, βglobulin, fibrinogen and γ-globulin.

Q.84 Which fraction will be absent in serum proteins and why?

Fibrinogen is absent in serum protein because fibrinogen is used up in clot formation in the separation of serum.

Serum = Plasma – Fibrinogen.

Q.85 Where is albumin synthesized in the body?

Albumin is mainly synthesized in the liver. The rate of synthesis is approximately 14.0 gm/ day.

Q.86 State the important functions of albumin.

  • Nutritive function.
  • Exerts low viscosity
  • Contributes 70 to 80% of osmotic pressure.
  • Plays important role in the exchange of fluids between blood and tissue.
  • Helps in the transport of several substances viz. FFA, unconjugated bilirubin, Ca++ and steroid hormones.
  • Binding of certain drugs: Sulphonamides, aspirin, penicillin are bound to albumin and carried in the blood.

Q.87 What is haptoglobin? What is its clinical importance?

Haptoglobin (Hp) is an α2-globulin synthesized in the Liver. Haptoglobin binds “free” Hb if present in blood. Average binding capacity of Hp irrespective of phenotype is approximately 100 mg/dl. Hp-Hb complex circulates in the blood and ultimately destroyed by RE cells.

Q.88 What is methemalbumin? What does it signify?

Normally blood does not contain methemalbumin. When the degree of intravascular (IV) hemolysis is rapid and severe, as may happen in incompatible blood transfusion, free Hb released exceeds the binding capacity of Hp. The free Hb combines with albumin to form methemalbumin, which can be detected by a sensitive test called Schumm’s test. Detection of methemalbumin points to IV hemolysis.

Q.89 What is Bence Jones protein? What is the clinical significance?

Bence Jones protein is an abnormal protein that occurs in the blood and urine of people suffering from a disease called multiple myeloma (a plasma cell tumor). It is a monoclonal light chain either “κ” or “λ” and excreted in the urine of multiple myeloma patients.

Q.90 How Bence Jones protein can be detected in urine in a clinical laboratory?

Presence of Bence Jones protein in urine can be identified easily by a simple “heat test”. Take urine in a clean test-tube and heat upto 50° to 60°C, when Bence Jones protein if present, are precipitated. But when heated further the precipitate dissolves again. Reverse occurs on cooling.

Q.91 What are the normal values of total proteins and differential proteins?

  • Total proteins: 7.0 to 7.5 gm%
  • Differential protein:
Gm% (by precipitation)  % of total proteins (by electrophoresis)
Albumin—3.7 to 5.3 50 to 70%
Globulins—1.8 to 3.6 29.5 to 54%
α1-globulins—0.1 to 0.4 2.0 to 6.0%
α2-globulins—0.4 to 0.8 5.0 to 11.0%
β-globulins—0.5 to 1.3 7.0 to 16%
γ-globulins—0.6 to 1.5 11.0 to 22.0%

 

 Q.92 What is the normal level of fibrinogen in plasma?

 0.2 to 0.4 gm% (200 to 400 mg%)

Q.93 What is the normal A: G ratio? In which condition the A: G ratio is reversed?

  • Normal A: G ratio is 2.5 to 1.0 (usually 2:1)
  • It is reversed characteristically in cirrhosis liver.

 Q.94 What are the types of hypergammaglobulinemias? Give a few examples.

Polyclonal gammopathies:
– Chronic infections, e.g. TB, kala-azar
– Chronic liver disease—cirrhosis liver
– Sarcoidosis
– Autoimmune diseases, e.g. rheumatoid arthritis, systemic lupus erythematosis (SLE)

Monolclonal gammopathies:
– Multiple myeloma
– Macroglobulinemia, etc.

Q.95 What is hypoproteinemia?

Decrease of total proteins below normal is called hypoproteinemia.

Q.96 State some causes of hypoproteinemia.

Hypoproteinemia can occur as follows:

  • Hemodilution:
    Water intoxication (overload), IV infusion of fluids. In this, both albumin and globulins are decreased and the A: G ratio remains unaltered.

  • Hypoalbuminemia:
    Conditions resulting in low albumin level, accompanied either by no increase in globulin or by an increase which is less than the fall in albumin. A: G ratio is decreased.
    Examples:
    – Loss through kidney: Nephrotic syndrome
    – Loss through G.I Tract: Protein-losing enteropathy, etc.

  • Hypogammaglobulinemia:
    – Conditions due to decrease in γglobulin.

Q.97 Name four transport binding proteins.

Transferrin (siderophillin): Binding and transport of Fe.

Thyroxine binding globulin (TBG): Binds and transport thyroxine (T4)

Transcortin: Binding and transport of cortisol.

Retinol binding protein (RBP): Binding and transport of retinol (Vit A).

Q.98 Which carbamoyl phosphate synthase is there in the urea cycle?

Carbamoyl phosphate synthase I.

Q.99 Name the hormones which favor tissue uptake of amino acids.

  • Insulin
  • Growth hormone (GH) and
  • Testosterone favors the uptake of amino acids by tissues. Hence these hormones are called “anabolic” hormones.
  • Estradiol stimulates selectively the uptake of amino acids by the uterus.
  • Epinephrine and glucocorticoids (GC) stimulate uptake of amino acids by the liver only.

Q.100 What is meant by nitrogen balance or nitrogen equilibrium?

  • In an adult healthy individual maintaining a constant weight, the amount of intake of ‘N’ in food (as dietary proteins mainly) will be balanced by an excretion of an equal amount of ‘N’ in urine (in the form of urea mainly, uric acid, creatine and to a small extent by amino acids) and
  • In faces (mainly as unabsorbed N)
  • Such an individual is then said to be in nitrogen balance or in nitrogen equilibrium.

Q.101 What is meant by positive nitrogen balance?

If the amount of nitrogen consumed exceeds the amount of nitrogen lost, the person is said to be in a positive nitrogen balance.
This happens in:
– Growing children
– Pregnancy
– Convalescence after illness/surgery, and
– After administration of anabolic hormones.

In positive nitrogen balance, the body puts on weight and N-intake will be greater than N-output since new cells are formed and N is retained as tissue proteins.

Q.102 What is meant by negative nitrogen balance?

A subject whose intake of N is less than the output of N is said to have a negative nitrogen balance. The person loses weight and the tissue proteins are catabolized to a greater extent than they are formed.
Such a situation occurs in:
– Old age
– Starvation
– Wasting diseases like tuberculosis
– Cancer
– Prolonged illness
– Postoperative conditions and burns.

Q.103 What non-protein nitrogenous substances are synthesized from amino acids? List them.

  • Creatine from glycine, arginine and methionine.
  • Heme from glycine and succinyl CoA.
  • Melanins from tyrosine.
  • Choline from serine through ethanolamine.
  • Purine nucleotides from glycine with 5- Phosphoribosylamine.
  • Pyrimidine nucleotides from aspartate and carbamoyl phosphate.

Q.104 What happens to the N-part of an amino acid?

In mammalian tissues, the α-NH2 group of amino acids, derived either from the diet or breakdown of tissue proteins, is first converted to NH3 and then to urea and excreted in urine. Humans are therefore called ureotelic.

Q.105 Why proteins are amphoteric in nature?

Proteins contain some free amino group and carboxyl group. It will give the tests of both acid and basic groups. Hence they are amphoteric in nature.

Q.106 Why proteins behave as buffers?

On either side of pI, Proteins behave as cations or anions depending upon the pH of the media and hence they behave as buffers.

Q.107 What is the denaturation of proteins?

Denaturation is defined as loss in secondary, tertiary, and quaternary (if present) structure of proteins on heating. Denaturation results in loss in the biological activity of the proteins.

Q.108 What is a nitrogen balance?

 If the ratio of N (protein) intake to N outtake is equal to 1, the person is in a Nitrogen balance.

 N intake  =1 
N out take