Active Sites – What’s Active Sites – What’s Happening?Happening?

• 1. Substrate is bound• 2. A reaction is being catalyzed . . . .• e. g.

Mechanism of Action: Mechanism of Action: 2 major 2 major

typestypes

1. Lock-and-key model of enzyme mechanism.

• 2. Induced-fit model

For HW- Skip #22.21 – too weird . . .

Mechanism of Action: Mechanism of Action: 2 major 2 major

typestypes • 1. Lock-and-key model of enzyme mechanism.• The enzyme is a rigid three-dimensional body.• The enzyme surface contains the active site.

HO

CH2OPO32-

OH

HHO

CH2OH

OHH

-D-Fructose-6-phosphate

O

HOOH

OH

CH2OPO32-

OH

-D- Glucose-6-phosphate

Phosphohexoseisomerase

HO

CH2OPO32-

OH

HHO

CH2OH

OHH

-D-Fructose-6-phosphate

O

HOOH

OH

CH2OPO32-

OH

-D- Glucose-6-phosphate

Phosphohexoseisomerase

HO

CH2OPO32-

OH

HHO

CH2OH

OHH

-D-Fructose-6-phosphate

O

HOOH

OH

CH2OPO32-

OH

-D- Glucose-6-phosphate

Phosphohexoseisomerase

1. 2.

1.

2.

NOTE: THE REACTION being Catalyzed(key)

(lock)

Mechanism of ActionMechanism of Action

HO

CH2OPO32-

OH

HHO

CH2OH

OHH

-D-Fructose-6-phosphate

O

HOOH

OH

CH2OPO32-

OH

-D- Glucose-6-phosphate

Phosphohexoseisomerase

HO

CH2OPO32-

OH

HHO

CH2OH

OHH

-D-Fructose-6-phosphate

O

HOOH

OH

CH2OPO32-

OH

-D- Glucose-6-phosphate

Phosphohexoseisomerase

2. Induced-fit modelThe active site becomes modified

to accommodate the substrate

1.

2.Induced fit – substrate changes

enzyme form/shape once in active site

Mechanism of ActionMechanism of Action• Figure 22.9 - Competitive inhibition.• the inhibitor fits into the active site, thereby preventing the substrate from entering.

Inhibitor got there 1st, Ha ha

Mechanism of ActionMechanism of Action• Figure 22.10 Noncompetitive inhibition.

• the inhibitor binds to allosteric site, ( not active site)• This changes conformation of the active site.

• The substrate no longer fits(properly).

Alosteric site

See p. 562-3 Chem. See p. 562-3 Chem. ConnectionsConnections

Mechanism of ActionMechanism of Action• Lock-and-key model & Induced-fit model emphasize the shape of the active site.

• However, Chemistry of active site is most important.

• Just 5 amino acids participate in active sites of:

≤65% of the enzymes studies to date.

• Examples: His > Cys > Asp > Arg > Glu.

• Four have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH).

• Which of the following is correct describing the

Induced-Fit Model of Enzyme action?

Substrates fit into the active site:

1. b/c they are exactly the same size and shape

2. By changing their size and shape to match those of the active site

3. By changing the size and shape of the active site Upon binding

Confirming your knowledgeConfirming your knowledge

• Enzymes are long protein chains, > 100 Amino acids

• The Active SITE contains only a few amino acids

• Explain why other amino acids are present and what would happen to the enzyme activity if significant changes were made to the enzymes Structure.

Challenge QuestionChallenge Question

Enzyme RegulationEnzyme Regulation

1. Feedback control

2. Proenzymes

3. Isoenzymes

Enzyme RegulationEnzyme Regulation1. Feedback control:1. Feedback control: regulation process where the product (of a series of enzyme-catalyzed reactions)

• inhibits an earlier reaction in the sequence.

• The inhibition may be competitive or noncompetitive.

A B C DE1 E2 E3

feedback inhibition

(path 1) e.g. cholersterol normally produced in liver, [<100mg/100mL ](path 2) if > [200mg/100mL] in plasma, liver stops producing . . .

A B C DE1 E2 E3

feedback inhibition

(path 1)

(path 2)

Enzyme Regulation (cont.)Enzyme Regulation (cont.)• 2. Proenzymes (zymogens):2. Proenzymes (zymogens): inactive form of enzyme

must have portion of polypeptide chain removed otherwise not active.

• e.g. trypsin, (a digestive enzyme)• synthesized/stored as trypsinogen, no enzyme activity.

• Active only after a six-amino acid fragment is removed

• Removal of amino acid fragment changes primary + tertiary structure, active form.

• 3. Isoenzyme:3. Isoenzyme: different form of the same enzyme ( two different forms catalyze the same reaction(s))

Challenge QuestionChallenge Question• Why not make fully active form of Enzyme(s) all the time? (e.g. trypsinogen trypsin)

The Allosteric EffectThe Allosteric Effect• Figure 22.12 Binding of regulator to a site other than the active site changes the shape of the active site.

Enzymes in MedicineEnzymes in Medicine• Enzyme assays useful in medical diagnosis.

Alanine aminotransferase (ALT)

Acid phosphataseAlkaline phosphatase (ALP)Amylase

Aspartate aminotransferase (AST)

Lactate dehydrogenase (LDH)Creatine phosphokinase (CK)Phosphohexose isomerase (PHI)

Enzyme Body Fluid Disease Diagnosed

Serum

SerumSerumSerum

Serum,Cerebrospinal fluidSerumSerumSerum

Hepatitis

Prostate cancerLiver or bone diseasePancreatic disease

Heart attack orhepatitis

Heart attackHeart attackHeart attack

Monitor levels 24hr after M.I. or (heart attack)