Metabolic Pathways & Enzyme Regulation
I. Overview of metabolism
Metabolism is the sum total of all of the enzymatically catalyzed
reactions occurring in the cell.
is divided into two phases
What is Intermediary Metabolism?
II. The Stages of Metabolism
The complex made simpler!
three stages of catabolism
The degradation of large macromolecules proceeds through a number of
consecutive molecular reactions organized into three major stages:
The three stages of anabolism
example: protein synthesis
Other Features of Anabolic and Catabolic Pathways
1. convergent and divergent
2. not the reverse
3. different locations
III. Nature of Enzyme Catalysis of Metabolism
The product of one reaction becomes the substrate of the next reaction
Three levels of complexity:
a. simple systems
b. multienzyme complex (pyruvate dehydrogenase reaction complex)
c. organized multienzyme complex (non cyclic photophosphorylation)
IV. Metabolic Regulation
Regulation: why do you need it?
1. Regulation by mass action
2. Regulation by enzyme activity
The key to metabolic regulation
A. Altering enzyme activity
1. substrate concentration
What about in the cell (in vivo)?
the importance of S concentrations near the Km
2. Km for substrate
Consider the following hypothetical pathway:
enzyme C (Km for substrate B = 5 mM)
A ® B
¯ enzyme D (Km for substrate B = 50 mM)
What does the Km tell you about enzyme-substrate affinity?
Consider the shape of the M-M curve and return to the hypothetical
Is the path to "C" is always favored because of the lower Km
for substrate of the enzyme which catalyzes that reaction?
What are they?
example: lactate dehydrogenase
catalyzes the following reaction:
pyruvate + NADH ---> lactate + NAD
5 different forms found in vertebrates that are combinations of two
polypeptide chains "M" and "H": thus, we have M4, M3H,
M2H2, MH3, H4 isozymes
M4 is mostly found in muscle tissue and has a low Km and high Vmax
H4 is mostly found in heart tissue and has a high Km
and low Vmax
Explain the differences in kinetic constants for each isozyme. Hint:
what are some differences between heart muscle and skeletal muscle?
4. Allosteric effectors
Allosteric effector binding site
What happens when an effector is bound to the allosteric site?
What sorts of molecules can be allosteric effectors?
Mechanism for allosteric regulation
an enzyme from glycolysis; reaction catalyzed:
fructose-6-P <---> fructose-1,6-diP
ADP & GDP (what is GDP?) are allosteric activators;
phosphoenolpyruvate is an allosteric inhibitor (this is an end-product of the pathway)
(Remember previous discussion regarding the ATP cycle and enzyme
regulation of ATP generating and ATP producing reactions?)
X-ray diffraction studies were done of the enzyme in active "R" and inactive "T" states
Thus, T--> R requires ADP or GDP
How have the chemical properties of the active site changed in the shift
from "T" to "R"?
Types of allosteric interactions
a. feedback inhibition
b. feedback stimulation
c. feedforward stimulation
another example of feedback inhibition:
aspartate transcarbamoylase (ATC ase): 12 polypeptide chains
this is the first step in the biosynthesis of CTP and UTP
CTP acts as a negative allosteric effector of ATCase
ATP acts as a positive allosteric effector of ATCase
Look at the kinetic curve for
ATCase with or without ATP and CTP.
What happens to the Km and Vmax values for aspartate under each condition?
How does this data help to show the nature of the allosteric effect for
5. Covalent modification of enzyme activity
a. enzymes are sometimes produced in an inactive form called a zymogen
pepsinogen--> pepsin + peptides (42 amino acids)
b. enzymes are modified by kinases and phosphatases
B. Regulating the Number of Enzyme Molecules
A complex topic not extensively covered in this cell biology
course (details to be found in genetics and molecular biology)
The number of enzyme molecules in the cell is a function of the
regulation of the genes that code for those enzymes.
Three components of gene control
What are the signals to which a specific gene responds?
At which level does regulation occur?
How is transcription regulated? Translation?
How do hormones affect protein synthesis?
How are parts of the DNA made available for transcription?