Properties

Understanding the physical and chemical properties of clinically relevant enzymes is essential for their accurate measurement and interpretation in the laboratory

General Properties

Proteins
- All enzymes are proteins. As such, they have characteristic amino acid compositions, molecular weights, and isoelectric points
Solubility
- Most enzymes are soluble in aqueous solutions, which is essential for their function in biological systems
Specificity
- Enzymes exhibit high specificity for their substrates and the reactions they catalyze. This specificity is determined by the unique three-dimensional structure of the enzyme’s active site
Catalytic Activity
- Enzymes act as catalysts, accelerating biochemical reactions without being consumed in the process
Sensitivity to Conditions
- Enzyme activity is sensitive to temperature, pH, and other environmental factors, which can affect their structure and function

LD or LDH

Definition: Lactate Dehydrogenase (LD or LDH) is an an enzyme that catalyzes the interconversion of lactate and pyruvate, with the reduction of NAD+ to NADH, and vice versa
Structure
- Tetrameric enzyme composed of four subunits
- Two types of subunits: M (muscle) and H (heart)
- Five isoenzymes: LDH-1 (HHHH), LDH-2 (HHHM), LDH-3 (HHMM), LDH-4 (HMMM), LDH-5 (MMMM)
Physical Properties
- Molecular Weight: Approximately 140,000 Da
- Isoelectric Point (pI): Varies depending on the isoenzyme
- Tissue Distribution:
  - LDH-1: Heart, red blood cells
  - LDH-2: Heart, red blood cells
  - LDH-3: Lung, other tissues
  - LDH-4: Liver, muscle
  - LDH-5: Liver, muscle
Chemical Properties
- Catalytic Activity: Catalyzes the reversible reaction between lactate and pyruvate
- Substrate Specificity: Lactate and pyruvate are the primary substrates
- Cofactor: NAD+ (nicotinamide adenine dinucleotide)
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 7.4
  - Inhibition: Inhibited by high concentrations of pyruvate and lactate
Clinical Significance
- Elevated levels indicate tissue damage
- Isoenzyme patterns can help identify the source of tissue damage

CK

Definition: Creatine Kinase (CK) is an enzyme that catalyzes the reversible transfer of phosphate from phosphocreatine to ADP, forming ATP and creatine
Structure
- Dimeric enzyme composed of two subunits
- Two types of subunits: M (muscle) and B (brain)
- Three isoenzymes: CK-MM (muscle), CK-MB (heart), CK-BB (brain)
Physical Properties
- Molecular Weight: Approximately 82,000 Da
- Isoelectric Point (pI): Varies depending on the isoenzyme
- Tissue Distribution:
  - CK-MM: Skeletal muscle
  - CK-MB: Heart muscle
  - CK-BB: Brain, smooth muscle
Chemical Properties
- Catalytic Activity: Catalyzes the reversible reaction between phosphocreatine and ADP
- Substrate Specificity: Phosphocreatine and ADP are the primary substrates
- Cofactor: Magnesium ions (Mg2+)
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 9.0
  - Inhibition: Inhibited by high concentrations of ATP
Clinical Significance
- Elevated levels indicate muscle damage
- CK-MB is a specific marker for myocardial damage

AST/ALT

Definition: Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT) are enzymes that catalyze the transfer of an amino group from an amino acid to a keto acid
Structure
- Monomeric enzymes
- AST has two isoenzymes: Cytosolic AST (cAST) and mitochondrial AST (mAST)
Physical Properties
- Molecular Weight: Approximately 45,000 Da (AST), 50,000 Da (ALT)
- Isoelectric Point (pI): Varies depending on the enzyme and isoenzyme
- Tissue Distribution:
  - AST: Liver, heart, muscle, kidney, brain
  - ALT: Primarily liver
Chemical Properties
- Catalytic Activity: Catalyze the transfer of an amino group
  - AST: Aspartate + α-ketoglutarate ↔︎ Oxaloacetate + Glutamate
  - ALT: Alanine + α-ketoglutarate ↔︎ Pyruvate + Glutamate
- Substrate Specificity: Aspartate and α-ketoglutarate (AST), Alanine and α-ketoglutarate (ALT)
- Cofactor: Pyridoxal phosphate (vitamin B6)
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 7.4
Clinical Significance
- Elevated levels indicate liver damage
- ALT is more specific for liver damage than AST
- AST:ALT ratio can help differentiate between different types of liver disease

GGT

Definition: Gamma-Glutamyl Transferase (GGT) is an enzyme that catalyzes the transfer of gamma-glutamyl groups from peptides to other amino acids or peptides
Structure
- Glycoprotein enzyme
- Located primarily on the cell membranes of liver, kidney, pancreas, and intestine
Physical Properties
- Molecular Weight: Approximately 85,000 Da
- Isoelectric Point (pI): Varies depending on the glycosylation pattern
- Tissue Distribution: Liver, kidney, pancreas, intestine
Chemical Properties
- Catalytic Activity: Catalyzes the transfer of gamma-glutamyl groups
- Substrate Specificity: Gamma-glutamyl peptides are the primary substrates
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 8.0
Clinical Significance
- Elevated levels indicate liver disease or biliary obstruction
- Useful for detecting alcohol-induced liver damage

Lipase

Definition: An enzyme that catalyzes the hydrolysis of triglycerides to glycerol and fatty acids
Structure
- Serine hydrolase enzyme
- Secreted by the pancreas
Physical Properties
- Molecular Weight: Approximately 48,000 Da
- Isoelectric Point (pI): Varies depending on the glycosylation pattern
- Tissue Distribution: Pancreas
Chemical Properties
- Catalytic Activity: Catalyzes the hydrolysis of triglycerides
- Substrate Specificity: Triglycerides are the primary substrates
- Cofactor: Colipase is required for activity
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 8.0
Clinical Significance
- Elevated levels indicate pancreatic damage (e.g., pancreatitis)
- More specific for pancreatic damage than amylase

Amylase

Definition: An enzyme that catalyzes the hydrolysis of starch to sugars
Structure
- Two major isoenzymes: Salivary amylase (S-amylase) and pancreatic amylase (P-amylase)
Physical Properties
- Molecular Weight: Approximately 54,000 Da
- Isoelectric Point (pI): Varies depending on the isoenzyme
- Tissue Distribution:
  - Salivary amylase: Salivary glands
  - Pancreatic amylase: Pancreas
Chemical Properties
- Catalytic Activity: Catalyzes the hydrolysis of starch
- Substrate Specificity: Starch is the primary substrate
- Cofactor: Calcium ions (Ca2+)
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 6.9
Clinical Significance
- Elevated levels indicate pancreatic damage (e.g., pancreatitis) or salivary gland disorders
- Less specific for pancreatic damage than lipase

ALP

Definition: Alkaline Phosphatase (ALP) is a group of enzymes that catalyze the hydrolysis of phosphate esters at alkaline pH
Structure
- Tetrameric enzymes
- Multiple isoenzymes, including liver ALP, bone ALP, intestinal ALP, and placental ALP
Physical Properties
- Molecular Weight: Approximately 80,000 Da
- Isoelectric Point (pI): Varies depending on the isoenzyme
- Tissue Distribution: Liver, bone, intestine, kidney, placenta
Chemical Properties
- Catalytic Activity: Catalyzes the hydrolysis of phosphate esters
- Substrate Specificity: Phosphate esters are the primary substrates
- Cofactor: Magnesium ions (Mg2+) and Zinc Ions (Zn2+)
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 10.0
  - Inhibition: Inhibited by phosphate and other inhibitors
Clinical Significance
- Elevated levels indicate liver disease, biliary obstruction, or bone disease
- Isoenzyme analysis can help identify the source of elevation

ACE

Definition: ngiotensin-Converting Enzyme (ACE) is an enzyme that catalyzes the conversion of angiotensin I to angiotensin II
Structure
- Zinc-containing metalloprotease
- Two isoforms: Somatic ACE and testicular ACE
Physical Properties
- Molecular Weight: Approximately 170,000 Da
- Isoelectric Point (pI): Varies depending on the isoform
- Tissue Distribution: Endothelial cells, kidney, lung, brain
Chemical Properties
- Catalytic Activity: Catalyzes the conversion of angiotensin I to angiotensin II
- Substrate Specificity: Angiotensin I is the primary substrate
- Cofactor: Zinc ions (Zn2+)
- Sensitivity to Conditions:
  - Temperature: Optimal temperature is around 37°C
  - pH: Optimal pH is around 7.5
Clinical Significance
- Elevated levels suggest sarcoidosis or other granulomatous diseases
- Used to monitor treatment of hypertension with ACE inhibitors

Key Terms

Enzyme: A biological catalyst
Isoenzyme: Different forms of the same enzyme
Substrate: The molecule acted upon by an enzyme
Cofactor: A non-protein molecule required for enzyme activity
Active Site: The region of the enzyme where the substrate binds
Catalysis: The acceleration of a chemical reaction by a catalyst
Hydrolysis: The chemical breakdown of a compound due to reaction with water
Spectrophotometry: A method to measure the absorbance of light by a solution
Amylase: An enzyme that catalyzes the breakdown of starch into sugars
Lipase: An enzyme that catalyzes the breakdown of fats into fatty acids and glycerol
Kinase: An enzyme that transfers phosphate groups to a molecule (phosphorylates). Most kinases act on hydroxyl groups of serine, threonine, and tyrosine residues of proteins
Phosphatase: An enzyme that removes a phosphate group from a molecule (dephosphorylates)
Decarboxylase: An enzyme that removes a carboxyl group from a molecule
AST: Aspartate Aminotransferase
ALT: Alanine Aminotransferase
GGT: Gamma-Glutamyl Transferase
ALP: Alkaline Phosphatase
LD/LDH: Lactate Dehydrogenase
CK: Creatine Kinase
ACE: Angiotensin-Converting Enzyme
Sarcoidosis: A disease characterized by the growth of tiny collections of inflammatory cells (granulomas) in any part of your body

Physiology

Procedures