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Lecture 20 Enzymes and Vitamins
Enzymes are catalysts. They lower the the activation energy of reaction so that they can
take place rapidly at physicological temperatures and generally physicological pH.
They are present in very small amounts, they are generally proteins and often contain a
metal
Enzymes are generally very specific; they catalyze some specific chemical process; in
order to perform their function, they often need specific substances called co-factors.
If the body does not produce this material, it must be present in the diet; these
cofactors are called vitamins.
Vitamins are classified on the basis of their solubility: fat or water soluble vitamins
Water Soluble Vitamins
Some Water Soluble Vitamins
Fat Soluble Vitamins
The most prevalent form of vitamin D is vitamin
D3, or cholecalciferol. Technically, this is not a
vitamin, because it is not required in the diet. In
skin, vitamin D3 is synthesized from 7dehydrocholesterol by the ultraviolet rays from
sunlight. In regions of limited sunlight, vitamin D3
is added to milk products to avoid a vitamin D3
deficiency. Its function in the body is to regulate the
absorption of phosphorus and calcium during bone
growth.
Enzymes may recognize and catalyze
a single substrate
a group of similar substrates
a particular type of bond
Isoenzymes
catalyze the same reaction in different tissues in the body;
lactate dehydrogenase (LDH), which converts lactate to pyruvate, consist of five
isoenzymes;
they can be used to identify the organ or tissue involved in damage or disease
lactate dehydrogenase have one form more prevalent in heart muscle and another
form in skeletal muscle and liver tissue
lactate dehydrogenase (LDH) is an enzyme made up of two polypeptide subunits,
Diagnostic enzymes
can determine the amount of damage
in tissues;
elevated levels may indicate damage
or disease in a particular organ;
levels of the enzyme creatine
kinase (CK), lactic dehydrogenase
(LDH), and aspartate transaminase
(AST) are elevated following a
heart attackare used to determine
the severity of the attack
Zymogens (proenzymes)
are inactive forms of enzymes;
they are activated when one or more peptides are removed
Example: The zymogen proinsulin is converted to its active form, insulin, by removing a small
peptide chain
Why is this necessary?
Digestive enzymes are produced as zymogens in one organ and transported to another,
such as the pancreas, when needed activated by removing small peptide sections
Effect of temperature on enzyme activity
Effect of pH on enzyme activity
Effect of enzyme concentration on rate
Effect of substrate concentration on enzyme activity
If a similar substrate binds to an enzyme but does not cause a reaction, the turnover rate of the
enzyme is sharply decreased. This is reversible competitive inhibition and how how many
drugs work.
In irreversible inhibition, the drug forms a covalent bond with the enzyme and removes it from
circulation. Many antibiotics work in this fashion.
Penicillin
R groups in penicillin
the transpeptidase in bacteria is used in a
step to form a cell wall (carbohydrate)