1. Enzymes
Homeostasis:
property of living organisms to regulate their internal environment, maintaining stable, constant condition
*Occurs by multiple adjustments controlled by related
regulation mechanisms
Metabolism:
all chemical reactions occurring constantly in cell that maintain homeostasis
Enzyme:
protein, site of a chemical reaction, can catalyze (speed up) reaction without being used up or changed
*Holds reactant molecules together long enough for them
to react

2. How Enzymes Work Substrate =
reactant(s) in an enzyme catalyzed reaction
Equation for an enzyme-catalyzed reaction:
Enzyme + Substrate => Enzyme/Substrate Complex => Enzyme + Product
(E + S => ES => E + P)
place where substrate(s) bind on enzyme
Active Site =
ENZYMES ARE MADE UP OF:
Apoenzyme:
protein part, gives enzyme specificity of what reaction it will catalyze
Coenzyme:
non-protein part, aids reaction by accepting or donating atoms (accepting= reduction donating = oxidation)
 Vitamins:
components for synthesizing coenzymes
*e.g. niacin (nicotinic acid), thiamine (B1) riboflavin (B2)

3. How Enzymes Work 1 = 3 = apoenzyme substrate 2 = 4 = coenzyme product
Induced Fit Theory:
Upon binding, enzyme undergoes slight change in shape to bind substrates: after reaction, ES complex separates, enzyme re-assumes original shape, ready to catalyze another reaction

4. How Enzymes Work Activation Energy: (Ea)
energy that must be supplied to cause molecules to react with one another
Lowering Activation Energy:
enzymes bring substrate molecules together, hold them long enough for reaction to take place
vs.

5. each reaction requires a specific enzyme
Metabolic Pathways
*orderly step-wise series of chemical reactions from
reactants to products where one reaction leads to next:
each reaction requires a specific enzyme
*not possible in biological systems to have single reaction
to produce complex molecules from simple reactants
(e.g. cellular respiration) many intermediates needed
*one pathway can lead to several others: intermediate
product of one pathway = starting reactant for other pathway
*having more than one step means more places where
overall reaction can be controlled

6. Factors Affecting Enzyme Activity
1. pH:
- each enzyme works optimally
in a preferred pH range (6-8 is common)
- if pH out of range (too high or low)
enzyme denatures (loses normal
configuration, ability to form
ES complex)
2. Temperature:
- ↓ temp slows rate of reaction
- very low temp does not normally
denature enzyme
- ↑ temp speeds up collision rate
of E + S (kinetic molecular theory)
- temp > 45 °C = enzyme denatured

7. Factors Affecting Enzyme Activity
3. Substrate Concentration:
- ↑ [substrate] = ↑ reaction rate
- after certain [substrate], rate will not ↑ anymore, enzymes
“saturated” with substrates (all active sites filled), cannot
work any faster
- ↓ [substrate] = ↓ rate of product formation

8. Factors Affecting Enzyme Activity
4. Enzyme Concentration:
- assume: typically millions more substrate molecules
than enzyme molecules
- therefore: ↑ [enzyme] = ↑ reaction rate
- ↓ [enzyme] = ↓ reaction rate
- rate will only level off if no more substrate left, not usual

9. Factors Affecting Enzyme Activity
5. Inhibitors:
molecules that bind to enzyme to prevent substrate binding to enzyme
a) Competitive Inhibition:
- molecule that looks like substrate competes for space at
active site, binds via induced fit, ↓ reaction rate
- inhibitor binding can be reversible or irreversible

10. Factors Affecting Enzyme Activity
b) Non-competitive Inhibition:
- inhibitor may look completely different from substrate:
binds to place on enzyme other than active site
- inhibitor binding causes enzyme to denature so substrate
cannot bind at active site
- binding maybe reversible or non-reversible

11. Factors Affecting Enzyme Activity
Inhibition Examples:
- metabolic pathways: intermediate or final product may be
reversible inhibitor of an enzyme earlier in pathway
- penicillin: irreversible competitive inhibitor of enzyme that
aids in bacterial cell wall synthesis
- hydrogen cyanide (HCN) = irreversible competitive
inhibitor of a mitochondria enzyme in humans
- Lead (Pb++), Mercury (Hg++), Cadmium:
heavy metals that are irreversible, noncompetitive
inhibitors of human enzymes

12. Metabolism Hormone - Thyroxin
Source Gland =
thyroid
Function =
protein hormone that regulates metabolism by controlling oxidation rate in cells
(oxygen + glucose = ATP)
Negative Feedback Loop:
Hypothalamus secretes
Thyroid Releasing Hormone (TRH)
Pituitary Gland secretes
Thyroid Stimulating Hormone (TSH)
Thyroid Gland secretes Thyroxin