Lecture 5: Enzyme Regulation Part 2

PRPP Synthetase (PRS) (Hint: 3)

Enzyme that catalyzes the first step in the synthesis of PRPP

Important for purine (adenine, guanine) production

Uses ATP; Ribose-5-phosphate -> PRPP

Isoenzymes (Hint: 4)

AKA isozymes

Enzymes that are encoded by different genes

Catalyze the same reaction by may display different regulatory properties

May be expressed in a tissue-specific or developmentally specific pattern

H Isozyme

Heart isozyme

M Isozyme

Muscle isozyme

Lactate Dehydrogenase (LDH) (Hint: 2)

Important enzyme for lactic acid fermentation

4 subunits

Protein Kinases (Hint: 3)

Modify proteins by attaching a phosphate to a serine, threonine, or tyrosine residue

ATP serves as the phosphate donor

OH groups can be modified through phosphates

Protein Phosphatases (Hint: 3)

Remove phosphates added by kinases

Hydrolysis reaction

Important because over phosphorylation can lead to diseases like cancer

Phosphorylation

A highly effective means of regulating the activities of target proteins

Acetylation (Hint: 3)

Addition of acetyl group

Lysine in proteins can be acetylated

Remove (+) charge

cAMP (Hint: 3)

Signal molecule

Only made in certain circumstances

2^nd messenger

Cushing’s Syndrome (Hint: 4)

Collection of diseases resulting from excess secretion of the hormone cortisol by the adrenal cortex

One cause is now known to a mutation that causes protein kinase A to be constitutive active (always on)

Mutation causes the catalytic (C) subunit to no longer bind the regulatory (R) subunit so that the enzyme is active even when cAMP is absent

Causes unregulated secretion of cortisol, which has various physiological effects, including suppression of the immune system and inhibition of bone growth

Cortisol (Hint: 2)

Stress hormone

Suppresses immune system

Phosphoproteomics

The study of the phosphoproteome

Phosphoproteome

All proteins that are modified by phosphorylation

Zymogens

Inactive precursors

Hemophilia A (Hint: 4)

AKA the royal disease

Sex-linked recessive disorder (only affects men) caused by inactive or missing factor VIII of the pathway

Factor VIII is not  protease, but should cause activation of factor X, the final protease in the pathway

Treated with recombinant factor VIII

Describe the Basis for Sigmoidal Curve for Allosteric Enzymes (Hint; 3)

Imagine an allosteric enzyme as a mixture of two Michaelis-Menten enzymes

One with a high value of K_m that corresponds to the T state (Inhibited; V-max lowered)

Another with a low value of K_m that corresponds to the R state

Describe an example of how the appearance of certain isozymes in the blood is a sign of tissue damage

Heart Attack Patient: Look for isozymes found in heart muscles but not in other tissues

Describe Isozymes of Lactate Dehydrogenase (LDH) (Hint: 3)

LDH has two different isozymes

Can be combination of both H and M

Every cell has different LDH requirements

Describe How Covalent Modification is a Means of Regulating Enzyme Activity (Hint: 3)

Enzymes can be modified by the covalent attachment of a molecule

Phosphorylation and acetylation are common modifications

Most covalent modifications are reversible

List the 3 key features of regulation by phosphorylation

The addition of the phosphoryl group alters electrostatic interactions (charge interactions; changes how amino acids nearby can react)

A phosphoryl group can form hydrogen bonds and ionic bonds

Phosphorylation and dephosphorylation can occur rapidly (most important point; cells can respond quickly; so fast they help mediate quick reactions)

How does cyclic AMP activate Protein Kinase A? (Hint: 4)

By altering the quaternary structure

cAMP stimulates protein kinase A (PKA) by binding to PKA’s regulatory (R) subunits, causing their dissociation from the catalytic subunits

The free catalytic (C) subunits are in the active form and can go phosphorylate transcription factors

Example: Epinephrine

Describe Epinephrine (Hint: 2)

Epinephrine (adrenaline; 1^st messenger) induces the “fight-or-flight” response in muscles

In muscle cells exposed to epinephrine, cAMP is synthesized

Describe how Exercise Modifies the Phosphorylation of Many Proteins (Hint: 3)

Recent research has shown that exercise results in the phosphorylation of nearly 600 different proteins

The kinases that catalyze these reactions include protein kinase A and AMP-activated kinase (AMPK)

These modifications affect many biological functions, including an increase in the ability to process fuels aerobically

Proteolytic cleavage plays a key role in what 2 examples of biochemical processes?

Activation of digestive enzyme

Blood clotting

Describe Chymotrypsinogen (Hint: 4)

Activated by specific cleavage of a single peptide bonds

The digestive enzyme chymotrypsin is synthesized as an inactive precursor called chymotrypsinogen

Multiple cleavages yields the mature enzyme, α-chymotrypsin

Digestive enzymes are dangerous if their activation is not controlled

Describe an example of a cascade of protease-activated zymogens that facilitates blood-clotting

Hemophilia is a defect in an early step in clotting

Why is Feedback Inhibition Important? PRPP Synthetase-Induced Gout (Hint: 4)

One of the causes of gout is a mutation in PRPP synthetase (PRS); Can’t bind purines (inhibitors); Defective feedback inhibition

Urate/uric acid is a final product of purine degradation

Gout is a joint disease in which excess urate crystals form in the fluid and lining of the joints; Overproduction of nucleotides

Painful inflammation results when immune cells engulf the crystals