Important Enzymes Flashcards
Glucokinase
Pathway: Sugar-trap
Increased transcriptional synthesis in response to high I/G ratio.
High Km, High Vmax
Hexokinase
Product: G-6-P
Pathway: sugar-trap
Increased transcriptional synthesis in response to high I/G ratio.
low Km, low Vmax
Phosphofructokinase-1 (PFK-1)
Fed state enzyme
Pathway: Glycolysis (Rate-limiting enzyme)
allosteric inhibitors: ATP, citrate
allosteric activators: AMP, F-2,6-BP
Increased transcriptional synthesis in response to high I/G ratio.
ATP-consuming
reciprocal regulation with FBPase-1
Pyruvate Kinase (PK)
Fed state enzyme Pathway: glycolysis Allosteric activators: F-1,6-BP (feed-forward) Increased transcriptional synthesis in response to high I/G ratio. --covalent modification: insulin: dephosphorylate = activate glucagon/epi: phosphorylate = deactivate ATP-generating
Lactate Dehydrogenase (LDH)
Anaerobic “11th” step of glycolysis
Pyruvate Carboxylase
Gluconeogenesis (therefore fasting state enzyme)
allosteric activators: AcCoA
Biotin-requiring coenzyme
PEP Carboxykinase
Pathway: Gluconeogenesis (ie fasting state, high I/G)
Increased transcriptional synthesis in response to high I/G ratio.
Who Rocks?
YOU DO!
Fructose-bisPhosphatase-1 (FBPase-1)
RATE-LIMITING!
Pathway: Gluconeogenesis (ie fasting state, high I/G)
Allosteric activators = ATP
Allosteric Inhibitors = F-2,6-BP, AMP
Reciprocal regulation with PFK-1 (via F-2,6-BP)
Phosphofructokinase-2/Fructose-bisPhosphatatse-2 (PFK-2/FBP-2)
–covalent modification:
insulin: dephosphorylate (active PFK-2 / inactive FBP-2 = high [F-2,6-BP] = allosteric activation of PFK-1 and allosteric inhibition of FPBase-1)
glucagon/epi: phosphorylate (deactive PFK-2 / active FPB-2 = decreased [F-2,6-BP] = release of allosteric inhibition on FBPase-1 and allosteric activation of PFK-1)
Pyruvate Dehydrogenase (PDHC)
TCA “Bridge”(3 enzymes, 5 coenzymes)
Product inhibition by NADH, AcCoA.
AcCoA can accumulate when not quickly oxidized by the TCA cycle; similarly, NADH accumulates when ETC is saturated or o2 is limited.
If PDHC activity is low, pyruvate will be shunted to lactate or to oaa.
Also COVALENTLY MODIFIED BY PDH Kinase and PDH Phosphatase (phosphorylation rule of thumb extends here, even without I/G)
PDH Kinase
INHIBITS PDHC via PHOSPHORYLATION
Pathway: TCA
Allosteric Activators = ATP, NADH, AcCoA
allosteric inhibitors = Pyruvate
PDH Phosphatase
ACTIVATES PDHC via DEphosphorylation
allosteric activators = Ca2+
(muscle contraction (Ca2+)/using energy = activates PDH phosphatse = DEphosphorylate/Activate PDHC = more AcCoA= more energy :)
Pyruvate carboxylase
Activated by high [AcCoA]
Pyruvate -> OAA
TCA Cycle “Priming”
Provides mechanism to “match” the levels of AcCoA with roughly equal amounts of OAA in order to catalyze the formation of citrizzle
Citrate Synthase (CS)
Pathway: TCA
Product inhibition by: Citrate (a high energy indicator)
(Citrate also allosterically inhibits PFK-1 (glycolysis) and activates FA synthesis (lets store this energy for a rainy day :)
Isocitrate Dehydrogenase
RATE-LIMITING (TCA Cycle)
NADH-Producing
Allosteric Activators = ADP, Ca2+
Allosteric inhibitors = ATP, NADH (high energy indicators)
alpha-Ketoglutarate Dehydrogenase (KGDHC)
pathway: TCA. NADH Producing
PRODUCT Inhibition by NADH & succinyl CoA
(allosteric activator = Ca2+)
Complex I
ETC
NADH passes it’s e- to this bad boy
This bad boy pumps protons
Passes his e- off to ubiquinone (coenzyme Q) which alley-oops the e- to complex III (within the phoshpolipid bilayer because ubiquinone is hydrophobic)
Complex II
ETC
FADH2 back door passes his e- to this complex II, who passes his e- to Ubiquinone (UQ), then UQ alley oops the e- to complex III, which dunks protons from the matrix to the intermembrane space (BOOYA!)
Complex III
ETC
Accepts e- from ubiquinone, who gets e- from complex I (NADH e- acceptor) and complex II (FADH2 e- acceptor)
Complex III pumps protons from the matrix into the intermembrane space. while passing e- to hydrophillic cytochrome, which finally passes e- to complex IV, which uses them to reduce O2 into H2O
Complex IV
Dr. Valla’s favorite enzyme
ETC
Consumes O2
Pumps Protons
Accepts e- from hydrophillic cytochrome C.
Allosterically regulated by ATP
(Primary regulation by energy state, similar to the TCA cycle) (add’l reg based on substrate availability (NADH, FADH2, O2) also dictates that ETC will run consistently during both fed and fasting state)
ATP Synthase (Complex V)
ATP-Generator! (ETC)
Complexes I, III and IV are able to pump protons, which create an electrochemical proton concentration gradient known as the mito membrane potential. When H+ ions are allowed to flow through ATP synthase, the flux is used t create ATP from ADP and Pi.
Transport Reliance on Proton Gradient
Pyruvate and inorganic phosphates are transported into mito via proton symport
ADP needs to be transported into the mito matrix where ATP can be re-synthesized.
ADP/ATP exchange (antiport) utilizes the voltage gradient (net outward movement on one negative charge (on ATP 4-) is favorable.
Mitochondrial protein import also depends on the mito membrane potential.
UDP-Glucose pyrophosphorylase
Activation (step 1) of Glycogenesis
Synthesizes UDP-glucose from G-1-P
UDP-glucose is the building block that is added to the growing glycogen granule
Branching Enzyme
Step 3 of Glycogenesis
forms branches of the strands, about every 8-10 residues, generating amylopectin structure
Glycogen Synthase
RATE LIMITING step 2 of glycogenesis
Regulation:
Dephosphorylation via protein phosphatase-1 (High I/G) activates glycogen synthase/glycogenesis while phosphorylation by PKA (low I/G) deactivates glycogen synthase (glycogensis)
Allosteric Regulation can override the hormonal regulation to respond quickly to the needs of the cell; elevated G-6-P will activate glycogenesis and inhibitch glycogenolysis
Glycogen Phosphorylase
RATE-LIMITING Step 1 (depolymerization) of Glycogenolysis
shortens strands via phosphorolysis of a1->4 bonds, down to 4 glycosyl residues (limit dextrin)
requires Pi and coenzyme pyridoxal phosphate (Vitamin B6)
Generates G-1-P from Pi and the shortening strand.
High I/G = dephosphorylation = inhbition of glycogenolysis (two targets, this enzyme as well as glycogen phosphorylase KINASE)
Low I/G = phosphorylation/activation of Glycogen Phosphorylation Kinase which activates glycogen phosphorylase (activating glycogenolysis)
Allosteric Regulation can override the hormonal regulation to respond quickly to the needs of the cell; elevated G-6-P will activate glycogenesis and inhibitch glycogenolysis
High energy levels (high ATP:AMP ratio) will inhibit glycogenolysis, especially in the muscle
High Ca2+/Calmodulin in muscle (from contractions, to feed glycolysis/prime TCA) and liver (via Epinephrine, to release more blood glucose) will activate glycogenolysis.
Debranching Enzyme
Step 2 of glycogenolysis
transfers 3 of the 4 glycosyls to nonreducing end of another chain and cleaves off the last glycosyl, releasing free glucose
Phosphoglucomutase
Step 3 (conversion) of G-1-P to G-6-P (reversible) exercising muscle lack G6Pase. G6P enters glycolysis (with saving 1 ATP since already phosphorylated). Consequently, muscle can never contribute to blood glucose, because it simply can't release free glucose into the blood :/
Glucose-6-phosphatase (G6Pase)
Glycogenolysis/Gluconeogenesis
Liver and kidney specific (muscle lacks this enzyme!)
G6Pase removes phosphate and frees glucose to enter the bloodstream.
