peptide bond bond formed between the carboxylic acid group of one amino acid
and the amino group of another amino acid.

primary protein structure sequence of amino acids in a polypeptide.

protein polymer linked together by peptide bonds that gives amino acids upon
hydrolysis. Proteins are found in plant and animal tissue.

secondary protein structure orientation of a peptide chain in space.

zwitterion molecule bearing both a positive and a negative charge. Neutral
protein molecules usually exist as zwitterions where the acidic carboxylate
hydrogen (–COOH) is transferred to the amino group (–NH₂ ) resulting in the
presence of –COO^– and –NH₃⁺ groups in the protein.

Enzymes

apoenzyme inactive, protein component of a holoenzyme.

coenzyme activator that converts an apoenzyme into an active enzyme.
enzyme biocatalyst protein produced by living cells; enzymes regulate cellular
reactions without themselves being altered or destroyed.

holoenzyme complete, activated enzyme.

metalloenzyme holoenzyme (active enzyme) produced by an apoenzyme and
a metal-ion activator.

molecular rate number of substrate molecules acted upon by a single enzyme
molecule per minute.

phenylketonuria build-up of phenylpyruvate in the body due to the absence
of the enzyme phenylalanine hydroxylase.

zymogen (also often called proenzyme) inactive form of an enzyme.

Common Student Misconceptions

1.   ‘‘Enzymes cause a reaction to occur.”
       Enzymes (or other catalysts) will not cause a reaction to ‘‘go” if that reaction
       won’t ‘‘go.” However, the enzyme does lower the potential energy barrier
       (activation energy) and permits the reaction to proceed at a much more rapid
       rate.

2.   ‘‘Enzymes (as found in dietary supplements, over-the-counter drugs,
      etc.) are effective when ingested.”
      Enzymes (and other proteins) are degraded in the stomach or intestine and
      are not absorbed. Only a few digestive enzymes are taken orally, and their
      effectiveness is questionable.

3.   ‘‘Only a few enzymes are needed in the body.”
      There are over 2,000 known biochemical reactions that require specific
      enzymes; thus there are over 2,000 different enzymes.

4.   ‘‘Enzymes are found only in animal cells.”
      Enzymes are found in both plants and animals, although they are all
      proteins. Turnips and potatoes, for example, contain the enzyme catalase
      that breaks down hydrogen peroxide into oxygen and water. The same
      enzyme is found in human blood.
 

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