Test: Enzymes - NEET PG MCQ

• Cyclooxygenase (COX), also known as prostaglandin H synthase
• Plays a role in the synthesis of prostanoids (prostaglandins, thromboxane, and prostacyclin)
• Exhibits two activities: cyclooxygenase and peroxidase
• Exists as two isoenzymes: COX-1 and COX-2

• NSAIDs:
  • Aspirin: Inhibits both COX-1 and COX-2
  • Indomethacin and ibuprofen: Inhibit cyclooxygenases by competing with arachidonate
• Coxibs: Selectively inhibit COX-2; however, some have been withdrawn or suspended from the market due to adverse side effects and safety concerns

• Completely inhibit the transcription of COX-2, but not COX-1

• Examples of lyases include: HMG CoA Lyase
• Argininosuccinate Lyase
• ATP Citrate Lyase
• Aldolase
• Fumarase.

• Monooxygenases incorporate one atom of an oxygen molecule into the substrate.
• Dioxygenases incorporate both atoms of an oxygen molecule into the substrate.

• Dehydrogenase
• Oxygenase
• Monooxygenase
• Dioxygenase
• Oxidase
• Catalase
• Peroxidase

• Monoamine Oxidase
• Cytochrome Oxidase
• Xanthine Oxidase

• Phenylalanine Hydroxylases
• 7 alpha Hydroxylases
• Cytochrome p450

• A + O₂ → AO₂

• Homogentisate oxidase
• Tryptophan Pyrrolase (Dioxygenase)
• Nitric Oxide Synthase

• Biotin serves as a coenzyme for carboxylation reactions.
• Pyridoxine acts as a coenzyme for decarboxylation.
• Thiamine functions as a coenzyme for the decarboxylation processes involving Alpha-ketoglutarate dehydrogenase and branched-chain ketoacid dehydrogenase.

Examples of competitive inhibition

Competitive inhibitors of enzymes are mostly drugs

Some competitive inhibitor which are not drugs are

Characteristics of competitive inhibition include:

• V remains a constant max
• K_m increases
• It is reversible.

Km (Michaelis Constant) represents the substrate concentration at which the reaction rate is half of Vmax for a given enzyme-substrate combination. It serves as an identifier for the enzyme:

• A higher Km indicates a lower affinity of the enzyme for the substrate.
• A lower Km signifies a higher affinity of the enzyme for the substrate.

Characteristics of competitive inhibition include:

• Km increases, resulting in reduced affinity.
• V_max remains unchanged.

Characteristics of noncompetitive inhibition include:

• Km remains constant.
• V_max decreases.

• Characteristics of noncompetitive inhibition:
• The inhibitor lacks any structural similarity to the substrate.
• It is irreversible.
• An excess of substrate does not eliminate the inhibition.
• The value of K_m remains unchanged.
• The maximum velocity (V_max) decreases.

• Competitive inhibition - Km rises while Vmax stays unchanged.
• Noncompetitive inhibition - Km remains constant, but Vmax decreases.
• Uncompetitive inhibition - Both Km and Vmax decline.

• Phosphorylation
• Dephosphorylation
• ADP Ribosylation
• Methylation
• Acetylation

• Zymogen activation
• Partial proteolysis

• Regulation of Enzyme Quality (Intrinsic Catalytic Efficiency)
• Allosteric Regulation
• Covalent Modification

• Phosphorylation/dephosphorylation (the most prevalent covalent modification)
• Methylation
• Adenylation
• ADP ribosylation
• Acetylation

• Control of Enzyme Synthesis through Induction and Repression
• Control of Enzyme Degradation

Serine proteases are a type of proteolytic enzyme characterised by the presence of serine at their active site. The amino acid triad located in the active site of serine proteases consists of:

• Ser
• His
• Asp

Examples of serine proteases include:

• Chymotrypsin
• Trypsin
• Elastase
• Thrombin
• Plasmin
• Complement Factors X and XL

Serine proteases exhibit variations in substrate specificity:

• Trypsin cleaves basic amino acids, such as Arg and Lys.
• Chymotrypsin cleaves hydrophobic, bulky amino acids like Trp, Tyr, and Phe.
• Elastase cleaves small, neutral amino acids such as Alanine and Glycine.

Enzymes as markers of organelle and membranes
Enzymatic markers of different membranes

Isoenzymes facilitate identical reactions. For instance, LDH₁-5 all convert pyruvate to lactate.

• They exhibit distinct quaternary structures.
• The subunits in LDH₁ differ from those in LDH₂.
• The tissue distribution of each isoform varies.
• The enzyme names and numbers may also differ.

Functional enzymes refer to enzymes that serve specific roles within the plasma. Examples include:

• Coagulation Factors
• Lipoprotein Lipase

Non-functional enzymes do not have a specific role in serum. They:

• Are released from tissues due to normal wear and tear
• Exhibit very low levels in serum
• Increase in serum levels during tissue injury, aiding in the diagnosis of the injury site

Examples of non-functional enzymes include LDH, Creatine Kinase, and Alkaline Phosphatase.

Estimation of blood creatinine can be conducted using two methods:

• Chemical Method: Based on Jaffe's Test
• In an alkaline medium, creatinine produces a red-coloured tautomer of creatinine pi crate, which is quantified colourimetrically.
• This technique can be automated in autoanalysers, and a kinetic method can also be employed.
• The kinetic Jaffe method is more precise than Jaffe's original method.
• Enzymatic Method:
• This method utilises two enzymes: Creatininase or Creatinine Deaminase.
• It is more specific and does not suffer interference from ketones, bilirubin, or glucose.
• Consequently, it measures creatinine with greater accuracy.

I test Function Assessed major Ł uuty [ I Serum bilirubin levels (total and conjugated) as an indicator of the liver's ability to conjugate and eliminate bilirubin (conjugation and excretory function). It assists in the differential diagnosis of jaundice.

• Total serum protein and albumin: This measures the biosynthetic function of the liver, since the liver is the main site for synthesising most plasma proteins. It also indicates the severity of chronic liver disease.

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I Test Aspect of Liver Function Assessed Major Utility:

• Prothrombin time: This assesses the biosynthetic function of the liver, as several coagulation factors are produced there. It indicates the severity of acute liver disease.
• Serum enzymes:
• a. Aspartate transaminase (AST): This acts as a marker for injury to hepatocytes that contain high levels of AST. The activity of serum AST and ALT are early signs of liver damage and help monitor treatment response.
• b. Alanine transaminase (ALT): This serves as a marker for injury to hepatocytes that contain high amounts of ALT.
• c. Alkaline phosphatase (ALP): This indicates biliary obstruction and assists in diagnosing blockages in the biliary tract.
• Blood ammonia: This reflects the liver's capacity to detoxify ammonia. Levels are elevated in liver cirrhosis with portal hypertension and in disorders of the urea cycle.