Enzymes & Factors Affecting Respiration | Biology for ACT PDF Download

Respiration

Factors Affecting Cell Respiration

(1) Temperature

• Optimum temp. for respiration is between 20-35°C. Maximum temp. is around 45°C.
• At low temp respiration is low due to inactivation of enzymes (Freeze preserve the food) while at very high temp. decrease, as enzyme denatured. Q10 = 2 to 3 for respiration.
• Production of potato crop is high on the hill areas due to low temp. throughout year.  CPRI(Central Potato Research Institute) is situated at Kufri-Shimla (H.P.)

(2) Oxygen

• The inhibition of anaerobic respiration by O₂ concentration is called as Pasteur's effect.
• The minimum amount of oxygen, at which aerobic respiration takes place & anaerobic respiration become extinct is called as extinction point.
• Oxygen concn. at which both aerobic & anaerobic respiration take place simultaneously is called as transition point.

(3) CO₂

•  If CO₂ concentration increases, then rate of respiration decreases in plants, (because stomata get closed).

(4) Salts

If a plant is transferred from water to salt solution, it's respiration increases, this is known as salt respiration. Because absorption of ions requires metabolic energy.

(5) Hormones

• IAA, GA  & cytokinin increase the respiration rate.
• The rapid increase in rate of respiration during ripening of fruits and senescence of leaves and plant organs is called as "Climacteric Respiration". This rate is decrease after sometime. It is due to production of ethylene hormone.

(6) Light

Rate of respiration increases with increase in light intensity. Light controls the stomatal opening & influence on temp. and also produce respiratory substrates.

(7) Injury, Disease & Wounds

The respiration increases due to injury wounding & infection.

(8) Inhibitors

CN, azides, DNP (Dinitrophenol), CO, rotenone, antimycin, amytal. etc inhibit the respiration.

(9) Age

Rate of respiration is more in young cells. Rate of respiration at meristem apex is high.

(10) Water

Seeds are slow respiring part of plants, because dry seeds are deficient of H₂O.

Enzymes

"Enzymes are biocatalysts made up of proteins (except ribozyme), which increases the rate of biochemical reactions by lowering down the activation energy."

• Enzymes are proteinaceous, biocatalysts.
• Term enzyme was given by Kuhne. 
• First of all isolated & discovered by Buchner 
• Zymase (from yeast) was the first discovered enzyme. (Buchner) 
• The first purified and crystalized enzyme was urease (by J.B.Sumner) from Canavalia/Jack Bean (Lobia plant).
• Proteinaceous nature of enzyme was suggested by Northrop and Sumner.
• First discovered ribozyme was L19 RNAase by T.Cech from rRNA of a protozoan Tetrahymena thermophila and RNAase P or Ribonuclease P by Altman in prokaryotic cell (Nobel prize).

Additional Information of Enzyme

Characteristics of enzymes

1. All enzymes are proteins, but all proteins are not enzymes.

2. Enzymes accelerate the rate of reaction, without undergoing any change in themselves.

3. M.wt. of enzymes ranges from 6000 (bacterial  fd) to 46 lakh (Pyruvate dehydrogenase comp.)

4. Enzymes are colloidal substances, which are very sensitive to pH & temperature. Optimum temp. for enzymes is 20-35°C.

5. Most of enzymes are active at neutral pH, hydrolytic enzymes of lysosomes are active on acidic pH (5).

6. Enzymes are macromolecules of amino acids, which are synthesized on ribosomes under the control of genes.

7. All enzymes are tertiary & globular proteins (Isoenzymes quarternary protein)

8. Enzymes are required in very minute amount for bio-chemical reactions. Their catalytic power is represented by Michaelis Menten constant or Km constant and turn over number. "The number of substrate molecules converted into products per unit time by one molecule of the enzyme in favourable conditions is called turn over number." The maximum turn over number is of Carbonic anhydrase, is 360 lakh, for Catalase is 50 lakh, for flavoprotein is 50 & for lysozyme is 30 per  minute.

9. Enzymes are very specific to their substrate or reactions.

10. Enzymes lower down the activation energy of substrate or reactions.

Structure of enzyme 

(1) Simple enzymes→ They are made up of only proteins. eg. pepsin, trypsin, papain. 

(2) Conjugated enzymes→They are made up of a protein part & non protein part (cofactor).

(i) Co-enzymes – Co-enzymes are non-protein organic groups, which are loosely attached to apoenzymes.

They are generally made up of vitamins.

(ii) Prosthetic group – When non-protein part is tightly or firmly attached to  apoenzyme.

(iii) Metal activators/metallic factor :- Loosely attached inorganic co-factor. eg. Mn,Fe,Co,Zn,Ca,Mg,Cu

•  Active site :- Specific part of amino acid chain in enzyme structure at which specific substrate is to be binded and catalysed, known as active site. Active site of enzyme is made up of very specific sequence of amino acids, determined by genetic codes.

•  Allosteric site :- Besides the active site's, some enzymes posess additional sites, at which chemicals other than substrate (allosteric modulators) are bind. These sites are known as allosteric sites and enzyme with allosteric sites are called as allosteric enzymes. e.g. hexokinase, phosphofructokinase.

Terminology 

•  Endoenzymes – Enzymes which are functional only inside the cells.

•  Exoenzymes – Enzymes catalysed the reactions outside the cell Eg:- enzymes of digestion, some enzymes of insectivorous plants, Zymase complex of fermentation.

•  Proenzyme/Zymogen – These are precursors of enzymes or inactive forms of enzymes. eg. Pepsinogen, Trypsinogen etc.

•  Iso-enzymes – Enzymes having similar action, but little difference in their molecular configuration are called isoenzymes. 16 forms of a-amylase of wheat & 5 forms of LDH (Lactate dehydrogenase) are known.

These all forms are synthesised by different genes.

•  Inducible enzymes – When formation of enzyme is induced by substrate availability. eg. Lactase, Nitrogenase,b-galactosidase,

•  Extremozymes – Enzymes, which may also function at extremely adverse conditions e.g Taq polymerase

•  Abzymes – When the monoclonal antibodies are used as enzymes or reagents.

•  Biodetergents – Enzymes used in washing powders are known as bio-detergents eg. – amylase, lipase, proteolytic enzymes.

•  House keeping / constitutive enzymes – Which are always present in constant amount & are also essential to cell.

Nomenclature and classification 

•  Enzyme commission of IUB-1961 divides all enzymes into 6 major classes and also proposed an international code of 4 digits for each enzyme.

(I) Oxido-reductases :-These enzymes involve in oxidation-reduction reactions.

It involves 3 sub classes

(i) Oxidase (ii) dehydrogenase (iii) reductase e.g.- Cytochrome oxidase.

(II) Transferases :- These Enzymes transfer specific group from one substrate to another. e.g.- Transaminase, Hexokinase.

(III) Hydrolases :- These Enzymes involve in hydrolysis reactions with help of H₂O. e.g. Proteases, Lipases, Carbohydrases.

IV) Lyases :- Split the substrate molecule without water. These Enzymes splits the specific covalent bonds without hydrolysis or H₂O addition. e.g. Aldolase.

(V) Isomerases :- Rearrangement of molecular structure to form isomers.

(VI) Ligases (Synthases) :- Covalent bonding of two substrates to form a large molecule. e.g. Citrate synthetase, Ligase Mode of action of Enzyme 

(1) Lock & Key theory or template theory :-

• Given by Emil Fischer 

•  According to this theory active sites of enzymes serve as a lock into, which the reactant/substrate fits like a key. Supported by competitive inhibition. 

(2) Enzyme - substrate complex theory :-– Given by Henry explained by Michaelis & Menten

– Given by Henry explained by Michaelis & Menten

E+S → ESC → EPC → E+P

(3) Induced fit theory – Given by D.E. Koshland (1973-74) – Accn to this theory active site is not static, but it undergoes a conformational change which is induced by specific substrate. The active site has two groups, (1) Buttressing (supporting) group & (2) Catalytic group – Buttressing group is meant for supporting the substrate, while the catalytic group break the substrate into product.

Factors

(1) pH – Enzymes very sensitive to pH.

(2) Temperature – High temp inactivates enzyme causing their denaturation . They also get inactive at lower temp. Generally all enzymes better perform at body temp. of organism.

(3) Enzyme concentration – Increase in conc. of enzymes will increase the rate of enzymatic reaction till enough the substrate.

(4) Substrate concn – Increase in substrate concn increases the activity of enzymes until all the active sites of enzyme are saturated.

(5) Inhibitors/Enzyme inhibition :

(i) Competitive inhibitors or competetive inhibition and reversible type :-– These are substrate analogues, which bind to the active site of enzymes & enzymes get inhibited, such inhibition is called ascompetitive inhibition.
Eg. Succinic dehydrogenase is inhibited by its competitor malonate. – This is reversible inhibition. Malonate is known as substrate analogue of succinate.

•  Similarly sulpha drugs are substrate analogue of p-amino benzoic acid (PABA) used in folic acid synthesis in bacterical cells. Hence these drugs are used to kill bacterial cells.

(ii) Non-competitive inhibitors or non competitive inhibition and irreversible type :-

•  In this type of inhibition, inhibitor substance can bind simultaneously to an enzyme, other than it's active site and destroy the sulfhydryl (S – H) group of enzyme. Example :- Toxic metals, CO, CN poisoning of cytochrome oxidase.

•  Such inhibition are irreversible inhibition.

(iii) Non competitive & reversible type :- When inhibitor binds at allosteric site reversibly.

•  When product of biochemical reaction inhibits the enzyme action, it is known as product inhibition or retro inhibition or feed back inhibition.

•  The product may binds at allosteric site of allosteric enzyme then it is non-competitive, reversible, allosteric inhibition. Example : inhibition of hexokinase by glucose 6P.

•  Jacob & Monad. First discovered L-threonine dehydratase, inhibited by its product isoleucine.

•  In the allosteric modulation, chemical or products fits in allosteric sites & bring a change in shape of active site of enzyme.

•  Chemicals which bind at allosteric site of allosteric enzymes are known as allosteric modulators.

•  If allosteric modulator positively change the configuration of active site, then called positive allosteric modulationand if negatively change then callednegative allosteric modulationrespectively by +ve modulator (activators) and –ve modulator (inhibitors).

Ex. Phosphofructokinase inhibited by ATP, activated by AMP, ADP and Glucose 2, 6 BisP.

Ex. hexokinase inhibited by glucose - 6P and exhibits feed back inhibition but not Glucokinase.

•  All allosteric modulation are not feed-back inhibition.

"Km constant of an enzyme, is the concentration of substrate at which rate of reaction of that enzyme attains half of its maximum velocity". It is given by Michaelis & Menten. The value of Km should be lower for an enzyme.
Km exhibits catalytic activity of an enzyme.

•  Ki constant (Enzyme inhibitor complex dissociation constant) indicates the dissociation of enzyme inhibitor complex (EIC). of reversible inhibitors.

•  Km and Ki constant of an enzyme should be low.

Old NCERT Syllabus

Special Points of Respiration and Enzymes

NAD = Nicotinamide Adenine Dinucleotide

DPN = Diphosphopyridine Nucleotide

NADP = Nicotinamide Adenine Dinucleotide Phosphate

TPN = Triphosphor Pyridine Nucleotide

FAD = Flavin Adenine Dinucleotide

FMN = Flavin Mono Nucleotide

Co-A = Adenosine Triphospho Panto Thenylthio Ethanol Acetyl Amine

 ATP discovered by Lohman, while importance of ATP in metabolism by Lipman.

 1 gram of fat equals to 9.8 K.Cal.

Proteins = 4.8 K. Cal

Carbohydrate = 4.4 K. Cal (Old 3.8 K. Cal)

(Fat is energy rich respiratory substrate)

• Almost all enzymatic reactions are reversible type.

•  Cytochromes are Iron - porphyrin protein discovered by MacMunn (Termed by Keilin)

•  Entner - Doudoroff pathway, Occurs only in bacteria (Pseudomonas and Azotobacter for Carbohydrate - oxidation. (Bact. respiration) 1 ATP = 7.6 K.Cal. (now 1 ATP = 8.9 K. Cal.)

Important Co-Factor

Fe⁺⁺ = Cytochrome oxidase, catalase, peroxidase, aconitase

Cu ⁺⁺ = Cytochrome oxidase, tyrosinase

Zn⁺⁺ = Carbonic anhydrase, alcohol dehydrogenase

Mg ⁺⁺ = Hexokinase, glucokinase, pyruvate kinase, pepcase, Rubisco.

K ⁺ = Pyruvate kinase

Mn ⁺⁺ = arginase, ribonucleotide reductase, decarboxylase.

Mo = Nitrogenase complex, Nitrate reductase.

Se = Glutathione peroxidase

• When respiratory substrate is fats or proteins, then level of Hg rises in Ganong's respirometer, because more O₂ absorbed than CO₂ released.

•  In bacteria site of ETS is mesosome.

•  Respiration efficiency :

• 1 glucose = 686 Kcal. 38 ATP ×  7.6 Kcal.

Thus efficiency of aerobic respiration is 42% .

• For the complete oxidation of one glucose if, in option  38 or 36 ATP are not given, then the answer goes to 32 or 30 ATP.

Significance of respiration 

1. The energy released during respiration is used for the various metabolic processes.

2. Various chemical substances are formed in this process which are important for cellular components.

3. CO₂ released in this process maintains a balance in the atmosphere.

4. Complex insoluble food materials are converted into simple soluble molecules by this process.

5. It converts the stored (static)energy into more useful (kinetic) form.