These notes are intended to accompany lectures in Human Physiology by Dr. Peter King at Francis Marion University, Florence, SC 29502.

What is an enzyme?
A biological catalyst.
Virtually all are proteins (but not all proteins are enzymes).

ENZYME comes from greek meaning in yeast.
Named by Eduard Buchner in mid 19th century.
He showed glucose was converted to alcohol by the cell content of yeast.
Previously it was thought that some life force supplied the energy.

Enzymes lower the activation energy of a reaction.
This happens because of their structure.
The 3 dimensional shape has a physical presence but also has atoms that can attract atoms of other molecules.
Active sites in enzymes attract specific molecules.

Enzymes can bring 2 molecules together and facilitate their bonding
or
attract one molecule and cause certain of its bonds to break.

Enzymes are not altered by the reaction they facilitate.

Enzyme activity is effected by 3 important characteristics.
1. Temperature
2. pH
3. concentration

Cofactors and coenzymes effect activity of enzymes.
cofactors are usually inorganic ions
coenzymes are organic molecules
They are necessary for the facilitated reaction. They may change the conformation of the enzyme or take part in forming temporary bonds.

The complex chemical reactions in cells normally take place in an orderly fashion in "metabolic pathways".
Each step of a metabolic pathway is controlled by an enzyme.
Inactivate an enzyme and the pathway is inactivated.

Glycolysis is a pathway common to all cells, involving 8 enzymes.

List of enzymes and cofactors in glycolysis on page 81of text.
Note: Phosphofructokinase inhibited by high ATP

Remember when we talk about function of physiological system at the organismal level, it is operating at the cellular level.
Enzymes control the 1000s of biochemical reactions going on in the billions of cells in your body at all times.
Can you feel it happening?

Why do we need oxygen?

We need oxygen because it is the final receptor of electrons in the electron transfer chain in mitochondria.

Let's step back a little.

Glucose oxidation is a central pathway for cell metabolism.
It has 3 major stages
1. Glycolysis
2. Krebs cycle
3. Oxidative phosphorylation

Glycolysis takes place in the cytosol of the cell (so even bacteria use glycolysis). Glucose is broken dowmn to pyruvate and in the process 2 molecules of ATP are formed (ADP + Pi -> ATP).

Pyruvate moves into the mitochondria, is converted to acetyl CoA and the acetyl group (2Cs) enters the Krebs cycle, in the mitochondrial matrix. The process of the Krebs cycle produces 1 moleculeof ATP and 5 molecules of reduced enzymes, 4 NADH and 1 FADH₂.

NADH and FADH₂ deliver electrons to a molecule embedded in the inner membrane of the mitochondrion. Flavin mononucleotide is the first molecule in a group of molecular complexes that form the electron transfer chain. The electron transfer chain receives electrons and pass them along the chain. The flow of electrons are used to do work, i.e. create a H⁺ gradients. The H⁺ gradient is used to phosphorylate ADP to form ATP.

Oxygen combines with H⁺ and electrons in the mitochondrial matrix to form water. Oxygen is the final receptor of the electrons in the electron transfer chain. If oxygen is not present, electrons cannot flow, no H⁺ gradient is created and consequently no ATP is formed.

How do molecules and ions move around our body?

The correct function of our organs depend largely on having the correct components available to allow chemical reactions to take place. Multicellular organisms have many areas separated by cell membranes to allow compartmentalization of environments to provide unique conditions for certain reactions.

Molecules and ions move in and out of cells by 4 major process
Simple diffusion
Facilitated diffusion
Active transport
Exocytosis/endocytosis

Simple Diffusion
Diffusion is a process where a solute will move from an area of high concentration to an area of lower concentrion. Cell membranes are semi permeable meaning that some molecules can move through them, in particular small uncharged particles such as CO₂, O₂ and H₂O. These molecules will move through the membrane if a concentration gradient exists.

Facilitated diffusion
Some proteins form channels or holes in cell membranes or act as carriers. This can allow the movements of large or charged particles across a cell membrane. The protein channels or carrier proteins are generally very specific to a certain particle. The presence of a protein channel facilitates diffusion of the particle. The particle will move through the channel if a concentration gradient exists or an electrical gradient exists.

Active transport
Active transport uses ATP (energy) to move particles across membranes against concentration and electrical gradients. Active transport of particles is produced by proteins that act as carrier protein and enzymes. They generally dephosphorylate ATP and use the energy released to change shape. The shape change moves the particle across the membrane. The proteins are often called transport proteins or pumps, i.e. Na⁺/K⁺ pump.

Exocytosis/endocytosis
Membrane bound particles, e.g. vessicles, can bind to, and become incorporated into, a cell membrane, thus emptying their contents on the outside of the cell (exocytosis). The particles in the membrane nver actually cross the lipid bilayer but are expelled from the cell. In the reverse process, particles outside a cell membrane can be surrounded by the membrane and drawn into the cell (endocytosis).

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This webpage was created by Peter King. Please contact the the author with comments at pking@fmarion.edu.
Last edited July 20, 2010
http://people.fmarion.edu/pking/humanphys/enzymes.html
copyright Peter King.