Proteins
Introduced| English |
|---|
| protein/ˈprəʊtiːn/ |
| amino acids/əˈmiːnəʊ ˈæsɪdz/ |
| peptide bond/ˈpeptaɪd bɒnd/ |
| polypeptide/ˌpɒlɪˈpeptaɪd/ |
| primary structure/ˈpraɪməri ˈstrʌktʃə/ |
The workhorses of the cell
- If DNA is the instruction manual, proteins are the machines it builds.
- Enzymes, antibodies, muscle fibres, and channels in membranes are all proteins.
- They do more different jobs than any other kind of molecule.
- And every one of those jobs comes down to a precise shape.
Amino acids: 20 building blocks
- A protein 蛋白质 is a chain of small units called amino acids 氨基酸.
- There are just 20 different amino acids.
- Their order along the chain is set by a gene.
- Twenty letters, in any order and length, spell out millions of possible proteins.
The monomer that builds a protein is the…
Proteins are chains of amino acids — there are 20 different kinds.
Peptide bonds make the chain
- Amino acids join by condensation, forming a peptide bond 肽键.
- A chain of many amino acids is a polypeptide 多肽.
- Each bond releases a water molecule, just like other polymers.
- But a straight chain is not yet a working protein.
The bond that joins two amino acids is a ____ bond.
A peptide bond links amino acids into a chain called a polypeptide.
Folding into shape
- The chain does not stay straight — it folds into a precise 3-D shape.
- The order of amino acids is the primary structure 一级结构.
- Local coils and sheets, then the whole fold, then joined chains build the higher levels.
- That final folded shape is what lets the protein do its job.
The four levels of protein structure
Step up the levels — a chain of amino acids folds into a precise 3-D shape that decides the protein's job.
A protein's 3-D shape decides what job it can do.
Shape is everything: an enzyme's active site, an antibody's grip — all depend on the folded shape.
Select all true statements about proteins.
Nucleic acids store the genetic code, not proteins. The other three are correct.
A protein's shape is fragile. High temperature or the wrong pH breaks the bonds holding the fold, and the protein denatures — it unfolds and stops working. This is why a fever is dangerous and why cooking changes egg white forever.
What happens when a protein is denatured by high heat?
Heat breaks the bonds holding the fold, so the protein denatures — it unfolds and can no longer work.
Why shape is everything — the enzyme active site:
- An enzyme has a pocket shaped to fit one specific molecule.
- The right molecule slots in like a key in a lock.
- Denature the enzyme and the pocket loses its shape — the "key" no longer fits, and the enzyme is useless.
Proteins are chains of amino acids joined by peptide bonds into a polypeptide. The chain folds through several levels (starting with the primary structure) into a precise 3-D shape, and that shape decides the protein's function. Heat or wrong pH denatures it, destroying the shape and the function.