4.3 - Proteins

Structure and Elements of Proteins

• Proteins are complex compounds consisting of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N); most proteins also contain sulphur (S) and phosphorus (P).
• All proteins consist of one or more polypeptides.
• Each polypeptide consists of monomers called amino acids.
• A polypeptide can contain from 50 to thousands of amino acid molecules.
• About 20 types of amino acids occur naturally.
• Different amino acid sequences produce different types of proteins.

Dipeptides and Polypeptides

• Dipeptide formation: Two amino acids join through condensation, one water molecule is removed, and a peptide bond forms.
• $\text{Amino acid}+\text{amino acid}\stackrel{\text{condensation}}{\to }\text{dipeptide}+\text{water}$
• Polypeptide formation: Further condensation joins more amino acids into a polypeptide chain.
• Dipeptide breakdown: Hydrolysis adds one water molecule and breaks a dipeptide into two amino acids.
• $\text{Dipeptide}+\text{water}\stackrel{\text{hydrolysis}}{\to }\text{amino acid}+\text{amino acid}$
• Proteins or polypeptides are broken down by digestive enzymes into amino acids; the amino acids are reused to build proteins required by the body.

Importance of Proteins in a Cell

• Build new cells.
• Repair damaged tissues.
• Synthesise enzymes, hormones, antibodies, and haemoglobin.
• Form building materials such as keratin in skin, collagen in bones, and myosin in muscle tissue.
• Their breakdown provides energy for daily activities.