Important Enzymes

Question 1

Glucokinase

Answer 1

Pathway: Sugar-trap
Increased transcriptional synthesis in response to high I/G ratio.
High Km, High Vmax

Question 2

Hexokinase

Answer 2

Product: G-6-P
Pathway: sugar-trap
Increased transcriptional synthesis in response to high I/G ratio.
low Km, low Vmax

Question 3

Phosphofructokinase-1 (PFK-1)

Answer 3

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

Question 4

Pyruvate Kinase (PK)

Answer 4

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

Question 5

Lactate Dehydrogenase (LDH)

Answer 5

Anaerobic “11th” step of glycolysis

Question 6

Pyruvate Carboxylase

Answer 6

Gluconeogenesis (therefore fasting state enzyme)
allosteric activators: AcCoA
Biotin-requiring coenzyme

Question 7

PEP Carboxykinase

Answer 7

Pathway: Gluconeogenesis (ie fasting state, high I/G)

Increased transcriptional synthesis in response to high I/G ratio.

Question 8

Who Rocks?

Answer 8

YOU DO!

Question 9

Fructose-bisPhosphatase-1 (FBPase-1)

Answer 9

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)

Question 10

Phosphofructokinase-2/Fructose-bisPhosphatatse-2 (PFK-2/FBP-2)

Answer 10

–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)

Question 11

Pyruvate Dehydrogenase (PDHC)

Answer 11

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)

Question 12

PDH Kinase

Answer 12

INHIBITS PDHC via PHOSPHORYLATION
Pathway: TCA
Allosteric Activators = ATP, NADH, AcCoA
allosteric inhibitors = Pyruvate

Question 13

PDH Phosphatase

Answer 13

ACTIVATES PDHC via DEphosphorylation
allosteric activators = Ca2+
(muscle contraction (Ca2+)/using energy = activates PDH phosphatse = DEphosphorylate/Activate PDHC = more AcCoA= more energy :)

Question 14

Pyruvate carboxylase

Answer 14

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

Question 15

Citrate Synthase (CS)

Answer 15

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 :)

Question 16

Isocitrate Dehydrogenase

Answer 16

RATE-LIMITING (TCA Cycle)
NADH-Producing
Allosteric Activators = ADP, Ca2+
Allosteric inhibitors = ATP, NADH (high energy indicators)

Question 17

alpha-Ketoglutarate Dehydrogenase (KGDHC)

Answer 17

pathway: TCA. NADH Producing
PRODUCT Inhibition by NADH & succinyl CoA
(allosteric activator = Ca2+)

Question 18

Complex I

Answer 18

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)

Question 19

Complex II

Answer 19

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!)

Question 20

Complex III

Answer 20

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

Question 21

Complex IV

Answer 21

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)

Question 22

ATP Synthase (Complex V)

Answer 22

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.

Question 23

Transport Reliance on Proton Gradient

Answer 23

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.

Question 24

UDP-Glucose pyrophosphorylase

Answer 24

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

Question 25

Branching Enzyme

Answer 25

Step 3 of Glycogenesis

forms branches of the strands, about every 8-10 residues, generating amylopectin structure

Question 26

Glycogen Synthase

Answer 26

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

Question 27

Glycogen Phosphorylase

Answer 27

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.

Question 28

Debranching Enzyme

Answer 28

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

Question 29

Phosphoglucomutase

Answer 29

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 :/

Question 30

Glucose-6-phosphatase (G6Pase)

Answer 30

Glycogenolysis/Gluconeogenesis
Liver and kidney specific (muscle lacks this enzyme!)
G6Pase removes phosphate and frees glucose to enter the bloodstream.