Review of Basic Metabolic Principles
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Review of Basic Metabolic Principles. A. Energy Currency of the Cell. Metabolic pathways can be classified as either: energy generating (catabolic) …or energy utilizing (anabolic). The cell also uses two types of energy currency:. 1. Phosphate anhydrides (compounds with high

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Review of Basic Metabolic Principles

A. Energy Currency of the Cell

  • Metabolic pathways can be classified as either:

    • energy generating (catabolic)…or

    • energy utilizing (anabolic)

The cell also uses two types of energy currency:

  • 1. Phosphate anhydrides(compounds with high

    • phosphate transfer potential)

General: hydrolysis of a phosphate ester

+

+

ATP hydrolysis

ATP

+

+


  • ATP is the most commonly used compound with

    • high phosphate transfer potential

    • phosphoenolpyruvate & creatine phosphate

      • have enough energy to synthesize ATP

    • ATP can be used to synthesize glucose 6-

      • phosphate & similar compounds

  • GTP is sometimes used in place of ATP


ATP is called the energy currency of the cell

Catabolism

Anabolism

O2

Work

Heat

ATP

Foods

Biosynthesis

CO2 + H2O

[Energy]

  • Definitions:

    • catabolism is the production of energy from

      • food

    • anabolism is the utilization of energy to provide

      • heat, do work, or drive biosynthetic reactions

  • Reducing equivalents(compounds with high

    • electron transfer potential)

Foods → NADH and FADH2 → ATP for Biosynthesis

  • The reducing equivalents in the foods we eat…

    • are transferred to NADH and FADH2

    • NADH and FADH2 transfer their electrons to

      • the electron transport chain which..

    • uses the energy in those electrons to synthesize

      • ATP


  • However, that’s not quite true. In reality:

    • catabolic pathways produce NADH, FADH2, &

      • ATP

    • anabolic pathways utilize ATP, NADPH, & FADH2

B. Coenzymes

1. Definition

  • Coenzymes are small molecular weight

    • compounds that are:

    • necessary for the catalytic activity of

      • one or more enzymes

  • present in very small amounts compared

    • to the substrates of the enzyme

  • used over & over in a catalytic manner

    • most coenzymes exist free in solution

    • some coenzymes are bound to proteins

      • = prosthetic groups


    • 2. Coenzymes carry some biologically important

      • chemical group in an “activated” (high energy)

      • form so that it can be used in biosynthetic

      • reactions

    Example = coenzyme A (carries acyl groups)

    + CoASH

    + H20

    acetic acid

    acetyl CoA

    G = -7.5 kcal/mole

    + CoASH

    + H20

    acyl CoA

    carboxylic acid


    3. Coenzymes are sometimes derived from vitamins

    Some activated carriers in metabolism


    C. What Does a Metabolic Pathway Look Like?

    • many reversible reactions

    • a few irreversible reactions that drive the

      • pathway

    • the irreversible reactions are important!

    A

    B

    C

    D

    E

    F

    E1

    E2

    E3

    E4

    E5

    D. Control of Metabolism

    1. Controls Which Operate at a Cellular Level

    • regulation is by inhibitors & activators in cell

    a. Which metabolites usually regulate pathways?

    • The precursor of a pathway usually

      • stimulates the pathway

    +

    A

    B

    C

    D

    E

    • ii. The end product of a pathway usually

      • inhibits the pathway

    -

    A

    B

    C

    D

    E


    A

    B

    C

    D

    E

    +

    G

    H

    I

    • Energy utilizing & producing pathways

      • are often regulated on the basis of

      • ATP supply in the cell

    • Catabolic pathways are inhibited by

      • ATP and/or stimulated by ADP or AMP

    • Anabolic pathways are stimulated by

      • ATP and/or inhibited by ADP or AMP

    b. Where are pathways regulated?

    i. At or near a branch point

    D

    A

    *

    B

    *

    C


    ii. At 1st committed (irreversible) step

    *

    A

    B

    C

    D

    E

    2. Controls that operate at whole body level

    • sites of regulation similar, but mechanisms different

    • Peptide hormones (glucagon, epinephrine,

      • & insulin)

    • act via second messenger (eg, cAMP)

    • result in phosphorylation or dephosphoryl-

      • ation

    • alter enzyme activity

    • are fast acting

    b. Steroid hormones (cortisol)

    • travel to nucleus & bind to DNA

    • affect amount of enzyme made

    • are slower acting


    Pi

    O

    +

    OPi

    E. Glossary of Enzyme Names

    • Kinase: catalyzes the phosphorylation of some

      • metabolite, usually with ATP as the donor

    Example = glucokinase or hexokinase

    ATP + glucose → ADP + glucose 6-phosphate

    • Phosphatase: catalyzes the hydrolytic removal

      • of a phosphate group (also called

      • dephosphorylation)

    Example = glucose 6-phosphatase

    glucose 6-phosphate + H2O → glucose + Pi

    • Phosphorylase: catalyzes the phosphorolytic

      • cleavage of a bond (phosphate is the

      • attacking nucleophile)

    Example = glycogen phosphorylase


    • Hydrolase: catalyzes the hydrolytic cleavage of

      • some bond (water is the attacking nucleophile)

    • Often subclassified according to bond cleaved

      • Esterase: hydrolyzes ester bond

      • Peptidase: hydrolyzes peptide bond

      • Phospholipase: hydrolyses phospholipids

    • Dehydrogenase: catalyses oxidation-reduction

      • reactions by the transfer of hydrogens

      • (electrons). Generally use NAD+/NADH or

      • FAD/FADH2.

    Example = lactate dehydrogenase

    • Synthetase or synthase: catalyzes the joining of

      • two molecules to create a larger molecule

    Example: citrate synthase

    oxaloacetate + acetylCoA + H2O → citrate + CoASH

    (4 carbons) (2 carbons) (6 carbons)


    • Carboxylase: catalyzes the addition of CO2 to a

      • molecule. Uses biotin as a coenzyme

    Example = pyruvate carboxylase

    OAA + ADP + Pi

    pyruvate + CO2 + ATP + H2O

    • Transferase: catalyzes the transfer of a group from

      • one molecule to another. Subclassified

      • according to group transferred

    • Transaminase: transfers amino groups. Uses

      • pyridoxal phosphate as a coenzyme.

    Example = aspartate transaminase (AST)

    +

    +

    -ketoglutarate

    Aspartate

    Glutamate

    Oxaloacetate


    • Transaldolase: transfers aldehyde groups. Uses

      • thiaminepyrophosphate as a coenzyme

    aldehyde:

    • Transketolase: transfers ketone groups. Uses

      • thiaminepyrophosphate as a coenzyme

    ketone:

    • Acyltransferase: transfers acyl (carboxylic acid)

      • groups. Uses coenzyme A as a coenzyme

    acyl (carboxylic acid):

    Methyltransferase: transfers methyl groups.

    Decarboxylase: removes carboxyl groups as CO2

    Isomerase: converts from one isomer to another


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