Cofactors in Enzymatic Activity

Cofactors

• Enzymes rely on small molecules, called cofactors, for catalytic activity.
• Cofactors are non-protein molecules that act as helpers.
• Two classes of cofactors are inorganic metals and small organic molecules (coenzymes).

Cofactors - Organic Molecules

• Organic molecules: derived from vitamins
• Examples: Thiamine pyrophosphate, Flavin adenine dinucleotide (FADH), Nicotinamide adenine dinucleotide (NADH), Pyridoxal phosphate, Coenzyme A (CoA), Biotin, 5'-Deoxyadenosyl cobalamin, and Tetrahydrofolate (THF).
• Enzymes: examples include Pyruvate dehydrogenase, Monoamine oxidase, Lactate dehydrogenase, Glycogen phosphorylase, Acetyl CoA carboxylase, and Pyruvate carboxylase,
• Cofactor is a small organic molecule (coenzyme).
• Coenzymes bind loosely to the apoenzyme, forming a holoenzyme.

Cofactors - Metal Ions

• Metal Ions: examples include Zinc (Zn2+), zinc (Zn2+), magnesium (Mg2+), magnesium (Mg2+), nickel (Ni2+), molybdenum (Mo), and Manganese (Mn2+)
• Examples of enzymes : Carbonic anhydrase, Carboxypeptidase, EcoRV, Hexokinase, and Urease

Holoenzyme

• Holoenzyme is enzyme-cofactor complex
• Apoenzyme + cofactor = Holoenzyme
• A holoenzyme is active
• Necessary for enzyme activity

Coenzymes (Organic Cofactors)

• Coenzymes are small organic molecules
• Often derived from vitamins
• Can be bound tightly or loosely to the enzyme.
• Play a role in catalytic activity.
• Examples include Thiamine pyrophosphate (TPP), Flavin adenine dinucleotide (FAD), and Nicotinamide adenine dinucleotide (NAD).
• Many vitamins are essential as coenzymes for enzymes or provide components for their synthesis.

Zymogens

• Some enzymes are synthesized as inactive precursors called zymogens (or proenzymes)
• These inactive forms are activated by proteolytic cleavage.
• Zymogen activation is essential to prevent self-digestion.
• Zymogens are found in digestive enzymes (peptidases).

Enzyme Activation by Proteolysis

• Proteolytic cleavage is common in cellular physiology.
• Digestive enzymes are synthesized as zymogens in the stomach and pancreas.
• Examples include pepsinogen (activated to pepsin) and trypsinogen (activated to trypsin).
• The pancreas and stomach secrete zymogens to prevent the enzymes from digesting the cells that synthesize them.
• Zymogen activations occurs in the Golgi apparatus or when digestive enzymes are secreted into the organ lumen.

Secretion of Zymogen Granules

• Exocrine pancreas cells have zymogen granules that are visible microscopically.
• Zymogen granules are in darker-staining clusters called acini.
• Secretory cells surround small intercalated ducts.

Proteolytic Cleavage - Activation

• Proteolytic cleavage activates enzymes.
• No energy is required for cleavage.
• Process is irreversible.
• It does not involve allosteric or covalent modification,

Apoptosis

• Apoptosis is programmed cell death.
• It involves proteolytic enzymes called caspases, derived as procaspases.
• Activated caspases cause cell death.
• Apoptosis produces special fragments called apoptotic bodies, which are cleared by macrophages.
• Unlike necrosis, apoptosis is highly regulated and does not cause inflammation.

Enzyme Regulation - Allosteric Regulation

• Allosteric enzymes change shape when an effector binds.
• Effectors (modifiers/regulators) bind non-covalently.
• Substrate binding can alter enzyme affinity.
• Effector binding changes the maximum catalytic activity of the enzyme.

Enzyme Regulation - Allosteric Regulation Types

• Homotopic effectors: the substrate itself serves as an effector, activating or inhibiting other areas of the enzyme.
• Heterotropic effectors: effector is different from the substrate.

Enzyme Regulation - Covalent Modifications

• Covalent modifications involve adding or removing phosphate groups from specific amino acids.
• Kinases add phosphate groups using ATP.
• Phosphatases remove phosphate groups, often involving the deactivation of the enzyme.

Enzyme Regulation - Enzyme Synthesis

• Enzymes regulated by synthesis are those needed only at specific stages of development or under certain conditions.
• Induction or repression occurs over hours to days.