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Biochem Block Two
Metabolism
28
Biochemistry
Graduate
09/27/2012

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Term
Creatine
Definition

Tissues, especially skeletal muscle, contain this
-When ATP synthesis exceeds immediate needs

(high energy charge), creatine is converted to phosphocreatine.  When energy is needed (e.g., for a burst of activity), phosphocreatine is used to convert ADP to ATP. Phosphocreatine has a high energy

phosphate bond (~10.3 kcal/mol). 

Term
FAD
Definition

One or two high-energy electrons

can be captured by FAD, forming FADH• (a free

radical) or FADH2. FADH2 is oxidized by O2 in

mitochondria to yield 1.5 ATP.

Term
NAD
Definition

Two high energy electrons in the

form of hydride ion (H- = H:) can be captured by

NAD+, forming NADH. Reoxidation of NADH by O2

in mitochondria ultimately yields 2.5 ATP.

Term
Acetyl-CoA
Definition

A central metabolic intermediate and a high

energy species that can transfer acetyl groups
(CH3-C(=O)-) to other biological molecules or to

water. Other acyl groups can also be attached to CoASH to yield high energy thioesters

Term
Substrate channeling
Definition

When enzymes for sequential steps are grouped so that the intermediates do not diffuse away.
-Such physically associated enzymes

are called a metabolon. 

Term
Glut 1 & 3
Definition

Responsible for basal glucose uptake and are therefore found in nearly all mammalian tissues, particularly in neuronal cells and RBCs (GLUT1 only).  Their Km for 

glucose is 1 mM, so they are always saturated.

Term
Glut2
Definition
Expressed in liver, pancreatic β-cells and the basolateral side of intestinal cells.  
-It has a high Km for glucose (15-20 mM), which means glucose uptake will be proportional to plasma concentration.  This property causes glucose uptake to increase when blood glucose levels increase.
Term
Glut4
Definition
Has a Km of 5 mM and transports glucose into skeletal muscle and fat cells.  Insulin increases GLUT4 translocation to plasma membrane, accounting in part for the hormone’s effect on glucose uptake into those tissues. 
Term
Glut5
Definition
Present on the apical side of small intestine cells and functions mainly for fructose uptake.
Term
SGLT1
Definition
The sodium, glucose cotransporter (sodium/glucose permease), is expressed on the apical side of intestine cells and is responsible for glucose and galactose uptake. 
Term
Glycogen Phosphorylase
Definition

This catalyzes the phosphorolytic cleavage of the α(1-4) 

bonds of glycogen.
- The enzyme works only on non-reducing ends that are > 4 residues away from a branch point. 
- Glycogen phosphorylase contains an active site 

pyridoxal phosphate (PLP) residue that acts as a general acid-base catalyst (this is an atypical use of PLP).  

Term
Glucose-6-Phosphatase
Definition

 Located on the luminal surface of the endoplasmic reticulum (ER), so G-6-P is transported out to it, and glucose is then transported back into the cytosol and 

then out into the plasma.
-this reaction occurs in liver, but not in skeletal muscle 

Term
Debranching enzyme
Definition
 Transfers a block of 3 glucose units from a α(1-6) branch from one outer branch to another (4-α-D-glucanotransferase activity) and (ii) hydrolyzes (no Pi) the remaining α(1-6)-linked glucose unit (α-1,6-glucosidase activity).
Term
Phosphofructokinase-1 (PFK-1)
Definition

Catalyzes the committed and rate-determining

step of the pathway
 -catalyzes the conversion of fructose-6-phosphate to

fructose-1,6-bisphosphate (FBP)
-ATP acts as an inhibitor of the enzyme, its affect is to change the Km of the enzyme for F6P, its substrate.
-Citrate, a central intermediate in the tricarboxylic acid (TCA) cycle, also serves as a feedback inhibitor in a manner similar to that of ATP
-
AMP, ADP, and inorganic phosphate (Pi) act as allosteric activators (signaling a need for more ATP production) by competing with ATP for binding to the allosteric ATP site. 

 

Term
Fructose-2,6-bisphosphate
Definition

Most important allosteric activator of PFK-1
-The concentration of fructose-2,6-bisphosphate is hormonally regulated by the balance between insulin and

glucagon (in the liver).
-The effect of fructose-2,6-bisphosphate is to modulate the Km of PFK-1 for its substrate fructose-6-phosphate and its inhibition by ATP. 

Term
Phosphofructokinase-2 (PFK-2)
Definition

This enzyme forms fructose-2,6-bisphosphate from a small amount of fructose-6-phosphate
-shares a polypeptide chain with FBPase-2
-regulated allosterically by fructose-6-phosphate

and covalently by phosphorylation of a single serine residue within the regulatory domain by a protein kinase.  It is activated in its unphosphorylated form

Term
Fructose bisphosphatase-2 (FBPase-2)
Definition

F-2,6,-bP broken down to fructose-6-phosphate and Pi by the action of this enzyme, which is on the same polypeptide chain as PFK-2
-activities are reciprocally regulated allosterically by fructose-6-phosphate and covalently by phosphorylation of a single serine residue within the regulatory domain

by a protein kinase. 
-activated in its phosphorylated form 

Term
Hexokinase
Definition

Glucose is converted to a metabolically active form not recognized by GLUT transporters (G-6-P) in most cells by this kinase
-occurs in several tissue-specific isoforms but all are active toward a broad range of hexose sugars have a low Km for glucose (~0.1 mM), use Mg-ATP as the true nucleotide substrate, and are inhibited by

glucose-6-phosphate (an example of product inhibition). 

Term
Pyruvate Carboxylase
Definition

In the mitochondrion pyruvate is converted to oxaloacetate by this enzyme
-an ATP dependent enzyme
-used in gluconeogenesis to bypass the pyruvate kinase step of glycolysis, which is irreversible
-an allosterically regulated biotin-dependent enzyme
-Oxaloacetate cannot cross the mitochondrial membrane and must be reduced to malate by

mitochondrial malate dehydrogenase or converted to aspartate by aspartate aminotransferase (AAT). 

Term
Oxaloacetate
Definition

-during gluconeogenesis, this is synthesized by pyruvate carboxylase in the mitochondira to bypass the pyrukate kinase step of glycolysis
-cannot cross the mitochondrian membrane & must be reduced to malate by mitochondrial malate dehydrogenase or converted to aspartate by aspartate aminotransferase (AAT).
-Once in the cytosol, malate is reoxidized to

oxaloacetate by cytosolic malate dehydrogenase or Asp is reconverted to OAA by cytosolic AAT. 

Term
Glucokinase
Definition

Glucose is converted to G-6-P in liver and in pancreatic β-cells by this enzyme.  
-has a higher Vmax and also a higher Km for glucose than hexokinase (~10 mM).
-It is not inhibited by glucose-6-phosphate.

-Glucokinase expression is increased by insulin. 

Term
Phosphoglucomutase
Definition

Glycogen phosphorylase releases glucose-1-phosphate (G-1-P), but further metabolism requires glucose-6-phosphate (G-6-P). Phosphoglucomutase makes this

conversion.
-also used in the synthetic process, where it converts the opposite reaction of G-6-P to G-1-P 

Term
UDP-glucose pyrophosphorylase
Definition
Converts G-1-P to an activated UDP-glucose in the gluconeogenesis pathway.  
-utilizes a UTP to activate G-1-P
-reaction is readily reversible but the reaction is driven forward by the subsequent hydrolysis of pyrophosphate by pyrophosphatase.   
Term
Phosphoprotein phosphatase
Definition
Dephosphorylates pyruvate kinase in the liver to reactivate it after its phosphorylation by protein kinase.
Term
Protein Kinase
Definition
Increased in liver cells by glucagon signaling.  
-cAMP increases which binds to the regulatory units of PKA and releases active PKA from them.
-PKA then phosphorylates pyruvate kinase to inhibit glycolysis
Term
Glycogen Phosphorylase Regulation
Definition
Phosphorylase is a dimer that exists in two conformations, a T state (inactive) and an R state (active).  
-conformation of phosphorylase b (inactive, phosphorylated) is mostly controlled by the metabolites ATP, G6P, & AMP.  It is most often in T state physiologically.
-high AMP levels will convert it to the R state
-phosphorylase a (active, unphosphorylated) is most often in the R state & is unresponsive to ATP & G6P physiologically.  It remains in the R state unless there is a high glucose level.  
Term
Phosphorylase Kinase
Definition
Phosphorylation of glycogen phosphorylase is controlled by this enzyme, which is under the influence of insulin & glucagon.  
-glucagon stimulates phosphorylation of phosphorylase kinase, which activates it.  Phosphorylase kinase then phosphorylates glycogen phosphorylase, activating it.
-Ca2+ also activates this in muscle
-also phosphorylates glycogen synthase, inactivating it
-insulin stimulates phosphoprotein phosphatase, which dephosphorylates glycogen phosphorylase & phosphorylase kinase
Term
Phosphoprotein Phosphatase
Definition
Activated by insulin
-dephosphorylates glycogen synthase & phosphorylase kinase, inactivating both 
-also dephosphorylates glycogen synthase, activating it 
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