Block 2 Flashcards
K(eq)
Equation
K(eq) = products / reactants
∆G°
Equation
∆G° = -R • T • ln( Keq )
Value of R in kcal / (mol • K)
R= 1.98 x 10^(-3) kcal / (mol • K)
Thermodynamic control vs. kinetic control
- Thermodynamic control is changing the free energy of either the reactants or the products.
- Kinetic control is changing the activation energy of the transition state (affecting the kinetics, but not the energy of the reaction).
Major classes of enzymes (6)
- Oxidoreductase
- Transferases
- Hydrolases
- Lyases
- Isomerases
- Ligases
Enzymes tightly bind the ______
transition state
Hydrophobic enzyme microenvironments affect pKa how?
Increases pKa
Peptidases
Require these two steps
- Polarization of peptide carboxyl group to form an oxyanion
- Proximity of nucleophile to attack the carbonyl carbon
Carboxypeptidases
How is the oxyanion formed?
By a Zn2+ on the enzyme
This amino acid can act as a general acid or as a general base
Histidine
Serine proteases utilize this triad for covalent catalysis.
Are they adjacent in the peptide?
- Ser - His - Asp
* Not adjacent
How does a serine protease work?
- Histidine’s nitrogen acts as a base to abstract a hydrogen from serine
- Serine’s oxygen acts as a nucleophile to the substrate’s carbonyl carbon
- Histidine’s hydrogen acts as a conjugate acid to the (cleaved) substrates nitrogen
- Histidine’s nitrogen again acts a a base to abstract a hydrogen from water
- The hydroxide acts as a nucleophile to the carbonyl carbon
- Histidine again acts as an acid, donating a proton to serine’s oxygen (freeing the substrate)
Specific vs. general acid-base reaction
Specific uses water, general does not.
Lineweaver-Burk plots
Y intercept?
X intercept?
Slope?
A plot of 1/Vo vs. 1/[S]
• (set 1/[S] = 0) = 1/Vmax
• (set 1/vo = 0) = -1/Km
• The slope is Km/Vmax
Competitive inhibition
Mechanism?
Effect on Lineweaver-Burk plot?
- Inhibitor binds to enzyme
* Increases slope
Noncompetitive inhibition
Mechanism?
Effect on Lineweaver-Burk plot?
- Inhibitor binds to enzyme and enzyme-substrate complex
* Reduces Vmax
Uncompetitive inhibition
Mechanism?
Effect on Lineweaver-Burk plot?
- Inhibitor binds to enzyme-substrate complex
- Reduces Vmax
- Apparent Km is decreased
Cooperativity in allosteric binding changes the ____
Km
Monosacharides
Two kinds, based on location of carbonyl carbon
- Aldose
* Ketose
Epimer
Sugars with multiple chiral carbons, differing at only one
Diastereomers
Sugars differing at one or more chiral carbons
Enantiomers
Mirror image at all chiral atoms
Absolute configuration
Dextro (D) and Levo (L). Refers to the carbon furthest from the carbonyl carbon.
Cyclization of monosacharides
Intermediate for aldoses?
Intermediate for ketoses?
- Hemiacetal
* Hemiketal
Name for a 6-carbon cyclized aldose?
Name for a 5-carbon cyclized aldose?
- Pyranose
* Furanose
Anomeric carbon
Carbon 1. Not chiral until it cyclizes. Though, do to mutorotation it will exist in both forms. Termed alpha (-OH down) and beta (-OH up).
Glycosidic bond
Where anomeric carbon is bound to another sugar. There is then no mutorotation between alpha and beta forms.
Reducing sugar
Sugars with a free aldehyde (C1 of aldoses) can reduce metals. Polysaccharides become non-reducing sugars.
Advanced Glycation Endproducts (AGEs)
Formation
- Glucose forms a a Shiff’s base with Lysine
* Vicinal -OH leads to Amadori rearrangement
Energy charge
Equation?
Typical value?
- ( [ATP] + 1/2[ADP] ) / ( [ATP] + [ADP] + [AMP] )
* 0.80 - 0.95
Glucose-6-phosphate
Branch point for which pathways (3)?
- Glycolysis
- Gluconeogenesis
- Pentose phosphate
Plasma glucose
Normal range?
Hypoglycemia?
Critical?
- 80 - 100 mg/dL
- 60 mg/dL
- 40 mg/dL
GLUT1 and GLUT3
Expression?
Km? Why?
- Most cells of the body. Dependent cells include neurons (mostly GLUT3) and RBCs (GLUT1 only)
- Km = 1mM, so always saturated. These cells must take in glucose at a rate commensurate with their typical metabolic rate.
GLUT2
Expression?
Km? Why?
- Liver, ß-islet cells, basolateral side of intestinal cells
* Km = 15-20mM. Flux is driven by plasma glucose level.
GLUT4
Expression?
Km?
Regulation by insulin?
- Skeletal muscles and fat cells
- Km = 5mM
- Insulin increases GLUT4 translocation to membrane surface.
GLUT5
Expression?
Main function?
- Apical side of small intestine cells
* Mainly uptakes fructose
SGLT1
Expression?
Main function?
- Apical side of intestine cells
* Sodium / glucose (or galactose) co-porter
Glucose trapping
How does it work?
Hexokinase converts glucose to G-6-P, which cannot pass through GLUT channel proteins.
Glucokinase
Expression?
What is its similarity to hexokinase?
What is its difference from hexokinase?
- Liver and ß-islet cells
- Catalyzes glucose –> G-6-P
- Is not inhibited by G-6-P
Glycogen
Main glycosidic bond?
Branching glycosidic bond?
Approximate ratio?
- alpha(1-4)
- alpha(1-6)
- 10:1
Glycogen phosphorylase
Cleaves glycogen at alpha(1-4) glycosidic bonds into G-1-P
Phosphoglucomutase
Converts G-1-P into G-6-P
Glucose-6-phosphatase
Found only on liver ER (lumenal). G-6-P is transported into ER, converted to glucose, exported to cytosol, then exported into blood stream.
Debranching enzyme
Hydrolyzes glycogen at alpha(1-6) branch points, releases free glucose. It does this by first transferring 3 glucose residues to the other branch, and cleaving the last remaining glucose residue.
UDP-glucose pyrophosphorylase
Adds a UDP group to G-1-P. PPi hydrolysis commits this step.
Glycogen synthase
Adds UDP-glucose onto an existing glycogen chain, alpha(1-4) linkages only. Glycogen chain must be >4 units long though.
Glycogenin
The prime mover of glycogen synthesis. Protein auto-glycosylates itself, using UDP-glucose to attach glucose to a tyrosine residue, up to 8 units long.
Branching enzyme
Aka glucosyl alpha-4,6-transferase. When alpha(1-4) glucose chains reach at least 11 residues, branching enzyme can come in and move a 7 glucose unit chain to a 6 position on a nearby chain (the 6 position must be at least 4 units from nearest existing branch point).
Regulation of glycogen phosphorylase
- Phosphorylated into more active form by phosphorylase kinase (yielding phosphorylase a)
- De-phosphorylated into less active form by phosphorylase phosphatase 1 (yielding phosphorylase b)
ATP value of NADH
ATP value of FADH2
- 2.5 ATP
* 1.5 ATP