LECTURE 15
Allosteric enzymes have 3 important properties
:1. Allosteric enzymes consist of 2 or more identical subunits. (Allosteric enzymes are oligomeric.)
2. Allosteric enzymes can exist in 2 conformational states, referred to as R (active) and T (inactive).
Substrate (S) binding by the enzyme stabilizes the R-state conformation; Negative effector (I) binding by the enzyme stabilizes the T-state conformation.
The conformational change is cooperative: If S binding causes one subunit in the oligomer to adopt the R state, all other subunits immediately shift into the R-state conformation.
3. Allosteric enzymes display sigmoid (S-shaped) substrate saturation curves. Due to the fact that v µ [S]n, where n>1.
Negative allosteric effector (e.g., Ile) shifts the substrate saturation curve to the right. Net effect is decrease in v (at concentrations of S that are less than saturating).
Allosteric regulation is achieved through noncovalent interactions (binding) of substrates and effectors to the enzyme.
Covalent modification: the reversible covalent modification of an enzyme.
Enzyme phosphorylation is the most common form of covalent modification. Phosphorylation on either Ser-OH, Thr-OH, or Tyr-OH groups. Adding or removing a phosphate group (a bulky, heavily negatively-charged functional group) has dramatic effects on protein conformation.
Enzyme exists in 2 states, modified (phosphorylated) and unmodified (unphosphorylated), where one is active, other is inactive.
Enzyme phosphorylation is catalyzed by ATP-dependent protein kinases. Phosphorylated enzymes are dephosphorylated by phosphoprotein phosphatases.
The abundance of many protein kinases in cells is an indication of the great importance of protein phosphorylation in cellular regulation. It is estimated that the human genome encodes more than 1,000 different protein kinases.
HOW CELLS MAKE ATP
Life is work: In the sense that work is free energy; free energy is needed to sustain vital activities, which are endergonic (+D G). This energy is supplied by ATP hydrolysis (-D G).
organic molecules rich in potential energy
(derived from photosynthesis)
catabolic ¯ D energy =
pathway ¯ energy
for work and heat
degradation to simple waste products having less potential energy
catabolic vs. anabolic:
anabolic = biosynthesis, as in n(amino acids) ® protein
anabolism is energy-requiring
Cellular Respiration: Most prominent catabolic thoroughfare.
"respiration" because O2 is consumed and CO2 is produced.
C6H12O6 + 6O2 ® 6CO2 + 6H2O + energy
Cellular respiration is an oxidation/reduction reaction (redox rxn):
oxidation º e- loss
reduction º e- gain
Reductant/reducing agent (AH2) + oxidant/oxidizing agent (B):
AH2 + B ® A + BH2
Reductant AH2 loses e- (and H+) to become A; oxidant B gains e- (and H+) and becomes BH2.
Potential energy in an organic compound is related to high-energy electrons associated with H:
-CH2- > -CHOH- > HC=O > -COOH
fat carbo carbonyl acid
C6H12O6 + 6O2 ® 6CO2 + 6H2O + energy
CO2; H2O = waste products of cellular respiration and raw materials of photosynthesis
Although represented as a single reaction (C6H12O6 + 6O2 ® 6CO2 + 6H2O + energy), cellular respiration consists of many steps (>20), energy released in small packets.
Cellular Respiration - 3 Stages
1. Glycolysis
2. Citric Acid Cycle
3. Oxidative Phosphorylation (electron transport + chemiosmotic ATP synthesis)
Some ATP is made in 1) & 2); most is made in 3)
2 Basic Mechanisms For Making ATP:
1. Substrate-level phosphorylation: direct transfer of phosphate (phosphoryl group) from an organic phosphate to ADP to make ATP.
example: pyruvate kinase (PK):
PEP + ADP ® pyruvate + ATP
Thermodynamically the sum of:
(1) PEP + H2O ® pyruvate + Pi D G° = -14.8 kcal/mol
(2) ADP + Pi ® ATP + H2O D G° = +7.3 kcal/mol
sum: PEP + ADP ® pyruvate + ATP D G° = -7.5 kcal/mol
Pyruvate kinase couples (1) & (2).
1. Oxidative phosphorylation via e- transport and chemiosmotic ATP synthesis:
the basic principle: a series of inner mitochondrial membrane proteins (e- transport complexes) mediate a series of redox rxns in which e- transfer leads to H+ translocation across the inner mitochondrial membrane (from inside to outside). That is, these e- transport complexes act like proton pumps. Proton translocation creates an electrochemical gradient of H+ ions that is used by ATP synthase to drive the formation of ATP from ADP + Pi.
(We're talking here about glucose oxidation)
Glucose oxidation and NAD+:
In most of the oxidative steps in glucose oxidation, electrons are transferred to NAD+, as hydride ions (H:-, a pair of e- + a proton).
The electron donor (AH2) is an intermediate along the pathway of glucose oxidation. The enzymes involved are dehydrogenases.
The NADH thus formed is a high-energy source of electrons.