Hormone and Enzyme

What is an enzyme?

A biological catalyst that increases reaction rate without being consumed

Main function of enzymes?

Lower activation energy and speed up reactions

Do enzymes change ΔG?

No

Do enzymes change equilibrium constant?

No

What is the active site?

Region where substrate binds and catalysis occurs

What is an apoenzyme?

Protein portion of an enzyme

What is a cofactor?

Non-protein helper required for enzyme activity

What is a coenzyme?

Organic cofactor often derived from vitamins

What is a holoenzyme?

Apoenzyme plus cofactor

4 ways enzymes lower activation energy?

Bring substrates together, stabilize transition state, stress substrate bonds, create favorable microenvironment

Are enzymes consumed in reactions?

No

Define Vmax.

Maximum reaction velocity when enzymes are saturated

Define Km.

Substrate concentration at half Vmax

Low Km indicates?

High substrate affinity

High Km indicates?

Low substrate affinity

Effect of increasing substrate concentration initially?

Reaction velocity increases

Why does reaction velocity plateau?

Enzyme saturation

Shape of Michaelis-Menten curve?

Hyperbolic

At Km what percentage of enzymes are occupied?

Approximately 50%

What is competitive inhibition?

Inhibitor competes with substrate for active site

Effect of competitive inhibition on Km?

Km increases

Effect of competitive inhibition on Vmax?

No change

Can competitive inhibition be overcome with more substrate?

Yes

What is noncompetitive inhibition?

Inhibitor binds allosteric site not active site

Effect of noncompetitive inhibition on Vmax?

Vmax decreases

Effect of noncompetitive inhibition on Km?

No change

Can noncompetitive inhibition be overcome with more substrate?

No

What is uncompetitive inhibition?

Inhibitor binds enzyme-substrate complex only

Effect of uncompetitive inhibition on Km?

Km decreases

Effect of uncompetitive inhibition on Vmax?

Vmax decreases

Which inhibitor changes substrate affinity?

Competitive inhibitor

Which inhibitor decreases number of functional enzymes?

Noncompetitive inhibitor

Effect of increasing temperature initially on enzyme activity?

Increases activity

Why does very high temperature reduce enzyme activity?

Denatures enzyme

Optimum temperature for most human enzymes?

About 37°C

How does pH affect enzymes?

Alters ionization and protein structure

What happens at extreme pH?

Enzyme denaturation

Optimum pH of pepsin?

About pH 2

Optimum pH of trypsin?

About pH 8

How does substrate concentration affect enzyme activity initially?

Increases reaction rate

Why does increasing substrate eventually stop increasing rate?

Enzyme saturation

What is feedback inhibition?

End product inhibits earlier enzyme in pathway

Purpose of feedback inhibition?

Prevent overproduction of product

What is allosteric regulation?

Regulation by binding at site other than active site

What happens when allosteric regulator binds enzyme?

Conformational change occurs

Can allosteric regulators activate or inhibit enzymes?

Yes

What kinetics do many allosteric enzymes show?

Sigmoidal kinetics

Define hormone.

Chemical messenger secreted to affect target cells

Main classes of hormones?

Steroid, peptide/protein, amino acid-derived

Examples of steroid hormones?

Cortisol, estrogen, testosterone, aldosterone

Examples of protein hormones?

Insulin, glucagon

Are steroid hormones lipid-soluble or water-soluble?

Lipid-soluble

Are peptide hormones lipid-soluble or water-soluble?

Water-soluble

Define endocrine signaling.

Hormones travel through blood to distant targets

Define paracrine signaling.

Hormones act on nearby cells

Define autocrine signaling.

Hormone acts on same cell that secreted it

Which signaling type acts over longest distance?

Endocrine

Why can steroid hormones cross cell membranes?

They are lipid-soluble

Where are steroid hormone receptors located?

Intracellularly

Mechanism of steroid hormones?

Diffuse into cell, bind receptor, bind DNA, alter gene transcription

Are steroid hormone effects rapid or slow?

Slow

Are steroid hormone effects long-lasting or short-lived?

Long-lasting

Why can’t protein hormones cross membranes easily?

They are water-soluble

Where are protein hormone receptors located?

Cell membrane

How do protein hormones produce effects?

Via second messenger systems

Are protein hormone effects rapid or slow?

Rapid

Main second messenger used by glucagon?

cAMP

Main function of insulin?

Lowers blood glucose

Main function of glucagon?

Raises blood glucose

How does insulin lower blood glucose?

Increases glucose uptake and glycogen synthesis

How does glucagon increase blood glucose?

Stimulates glycogen breakdown and gluconeogenesis

What receptor type does insulin use?

Tyrosine kinase receptor

What receptor type does glucagon use?

G-protein coupled receptor

Competitive inhibition effect on Km and Vmax?

Increased Km, unchanged Vmax

Noncompetitive inhibition effect on Km and Vmax?

Unchanged Km, decreased Vmax

Uncompetitive inhibition effect on Km and Vmax?

Decreased Km and decreased Vmax

Steroid hormone receptor location?

Intracellular

Protein hormone receptor location?

Cell membrane

Steroid hormone solubility?

Lipid-soluble

Protein hormone solubility?

Water-soluble

Steroid hormone onset?

Slow

Protein hormone onset?

Fast

Steroid hormone duration?

Long-lasting

Protein hormone duration?

Short-lived

Do enzymes change equilibrium position?

No

Which inhibitor can be overcome by increasing substrate concentration?

Competitive inhibitor

Which inhibitor decreases catalytic capacity of enzyme?

Noncompetitive inhibitor

What does low Km always mean?

High affinity

Which hormones act through gene transcription?

Steroid hormones

Which hormones commonly use second messengers?

Protein/peptide hormones

Competitive inhibitor effect?

Increased Km, same Vmax

Noncompetitive inhibitor effect?

Decreased Vmax, same Km

Low Km means?

High affinity

Protein hormone receptor location?

Cell membrane

Insulin effect on blood glucose?

Decreases blood glucose

Glucagon effect on blood glucose?

Increases blood glucose

What happens at Vmax?

All enzyme active sites saturated

Main role of allosteric regulation?

Modifies enzyme activity via conformational change

Main role of feedback inhibition?

Prevents excess end-product formation

Main function of enzymes?

Lower activation energy