Chapter 1–2: Enzymes

What are enzymes?

Biological catalysts that speed up chemical reactions in living organisms without being consumed.

How do enzymes speed up reactions?

They lower the activation energy needed for the reaction.

What is the basic enzyme reaction?

Substrate is converted into product with the help of an enzyme.

Are enzymes consumed during reactions?

No, enzymes are released unchanged and can be reused.

What are most enzymes made of?

Proteins.

What are simple enzymes?

Enzymes composed only of amino acids.

What are conjugated enzymes?

Enzymes made of a protein part plus a non-protein prosthetic group.

What does enzyme specificity mean?

An enzyme usually binds and reacts with only one or a few specific substrates.

Give an example of enzyme specificity.

Arginase acts on L-arginine.

What does chymotrypsin do?

It breaks peptide bonds in proteins, especially after aromatic amino acids like tyrosine and phenylalanine.

What are endoenzymes?

Enzymes that act at the place where they are synthesized.

What are exoenzymes?

Enzymes produced in one place but acting in another place.

Give an example of an exoenzyme.

Amylase, which breaks down starch.

What are isoenzymes?

Enzymes that catalyze the same reaction but have different physical and chemical properties.

Why are isoenzymes clinically useful?

They help identify which organ or tissue is damaged.

What is the active site?

The specific part of an enzyme where the substrate binds.

What is the lock-and-key model?

The enzyme active site already has the perfect shape for the substrate.

What is the induced-fit model?

The enzyme changes shape slightly to fit the substrate better.

What are contact amino acids?

Amino acids in the active site that help bind the substrate.

What are catalytic amino acids?

Amino acids that directly participate in the chemical reaction.

What do structural amino acids do in enzymes?

They maintain the 3D shape and stability of the enzyme.

What is the Michaelis-Menten theory?

The theory that enzymes form an intermediate enzyme-substrate complex during the reaction.

What is the first phase of an enzyme reaction?

Formation of the enzyme-substrate complex.

What is the second phase of an enzyme reaction?

Product formation and release of the unchanged enzyme.

What does E + S ⇄ ES mean?

Enzyme and substrate reversibly form an enzyme-substrate complex.

What does ES → E + P mean?

The enzyme-substrate complex forms product and releases the enzyme.

Which factors influence enzyme reaction rate?

Temperature, pH, enzyme concentration, substrate concentration, activators, and inhibitors.

What is the temperature coefficient Q10?

The change in reaction speed when temperature increases by 10°C.

What happens to enzyme activity when temperature increases?

It increases until the optimum temperature is reached.

What happens above the optimum temperature?

The enzyme denatures and activity decreases.

What is optimum pH?

The pH at which an enzyme works with the highest reaction rate.

What happens to enzymes in very acidic or alkaline conditions?

They can denature irreversibly.

How does enzyme concentration affect reaction rate?

If substrate is sufficient, more enzyme causes a faster reaction.

How does substrate concentration affect reaction rate?

Rate rises with substrate concentration until enzymes become saturated.

What is Vmax?

The maximum rate of an enzymatic reaction.

What is Km?

The substrate concentration at which the reaction rate is half of Vmax.

What does low Km mean?

The enzyme has high affinity for the substrate.

What does high Km mean?

The enzyme has lower affinity for the substrate.

What are enzyme activators?

Substances that increase enzyme activity.

Give examples of enzyme activators.

Mg2+, Ca2+, Cu2+, Zn2+.

What are enzyme inhibitors?

Substances that decrease enzyme activity.

What is competitive inhibition?

The inhibitor resembles the substrate and competes for the active site.

What is non-competitive inhibition?

The inhibitor binds to a different site, not the active site.

What is acompetitive inhibition?

The inhibitor binds only to the enzyme-substrate complex.

What is allosteric inhibition?

An inhibitor binds to an allosteric site and reduces enzyme activity.

Why is allosteric inhibition important?

It helps regulate enzyme activity and prevents accumulation of products.

What is irreversible inhibition?

The inhibitor permanently blocks the enzyme.

Why can enzyme levels increase in blood?

Due to increased synthesis, membrane leakage, cell necrosis, impaired secretion, or impaired elimination.

Why does cell necrosis increase enzyme levels?

Damaged or dead cells release intracellular enzymes into blood.

What is enzyme half-life?

The time needed for enzyme activity to drop to half of its original value.

What is the half-life of AST?

About 12–22 hours.

What is the half-life of ALT?

About 37–57 hours.

What is the half-life of CK?

About 15 hours.

What is the half-life of alpha-amylase?

About 3–6 hours.

Why is enzyme half-life clinically important?

It helps doctors time tests and interpret enzyme results.

Which enzyme is typical for salivary glands?

Alpha-amylase.

Which enzyme is typical for bones?

Alkaline phosphatase.

Which enzyme is typical for the prostate?

Prostatic acid phosphatase.

Which enzymes are common in heart damage?

CK, LDH1, AST, ALT.

Which enzymes are common in skeletal muscle damage?

CK, LDH5, AST, ALT.

Which enzymes are common in liver damage?

LDH5, AST, ALT.

Which enzyme is important in bile duct disease?

ALP.

Which enzymes are important in pancreatic disease?

Lipase, alpha-amylase, AST.

How is enzyme activity measured in laboratory medicine?

By measuring the rate at which the enzyme catalyzes a reaction.

What can be followed to measure enzyme activity?

Product formation or substrate disappearance over time.

What is the single-point method?

Enzyme activity is measured at one specific time after stopping the reaction.

What is the kinetic method?

The reaction is monitored continuously or at intervals over time.

Why is the kinetic method useful?

It measures the change in absorbance over time and gives reaction rate.

What is the optical test based on?

NADH and NADPH absorb light around 340 nm, while NAD+ and NADP+ do not.

What happens to absorbance when NADH is oxidized?

Absorbance decreases.

What happens to absorbance when NAD+ is reduced to NADH?

Absorbance increases.

Which enzyme assay is a classic optical test example?

LDH activity determination.

What does LDH do in the optical test?

It converts pyruvate and NADH into lactate and NAD+.

What does decreasing absorbance at 340 nm show in LDH testing?

Oxidation of NADH and LDH activity.

What is one IU of enzyme activity?

The amount of enzyme that converts 1 micromole of substrate per minute.

What is one katal?

The amount of enzyme that converts 1 mole of substrate per second.

Which enzyme activity unit is more common in medicine?

IU.

Which enzymes are frequently measured in clinical labs?

AST, ALT, CK, alpha-amylase, GGT, LDH, ALP.

What does AST stand for?

Aspartate aminotransferase.

What does AST catalyze?

Reversible transamination between aspartate and alpha-ketoglutarate.

Where is AST mainly found?

Liver, heart, and skeletal muscle.

Which AST isoenzymes exist?

Cytoplasmic AST and mitochondrial AST.

When is AST physiologically elevated?

In newborns during the first days after birth.

What are indications for AST measurement?

Hepatobiliary disease, myocardial infarction, and skeletal muscle disease.

How high can AST rise in myocardial infarction?

More than 10 times above reference values.

When does AST rise after myocardial infarction?

After 6–8 hours.

When does AST peak after myocardial infarction?

After 18–24 hours.

When does AST normalize after myocardial infarction?

After 4–5 days.

What is the De Ritis quotient?

The AST/ALT ratio.

What does AST/ALT greater than 1 suggest?

Progressive liver disease.

What does AST/ALT less than 1 suggest?

Milder liver disease.

How high can AST rise in acute viral hepatitis?

About 50–70 times above reference values.

In acute toxic hepatitis, which is usually higher: AST or ALT?

AST is higher than ALT.

What happens to AST in chronic hepatitis?

It is normal to slightly elevated.

What happens to AST in obstructive jaundice and cirrhosis?

It is elevated.

What happens to AST in muscular dystrophy?

It is elevated.

What happens to AST in myositis?

It is slightly elevated.

Why is AST normal in myasthenia gravis?

Myasthenia gravis affects the neuromuscular junction, not muscle destruction.

What does ALT stand for?

Alanine aminotransferase.

What does ALT catalyze?

Transamination between alanine and alpha-ketoglutarate to form pyruvate and glutamate.