Proteins Made of Amino Acids Carry Out Many Cell Functions

Proteins are multifunctional biological polymers that consist of one or more polypeptide chains. The information carried by messenger RNA is translated to give a polypeptide chain. The linear sequence of nucleotides in the RNA (read in groups of three—i.e. codons) corresponds to the linear sequence of the amino acids that make up the polypeptide chain. That is, the mRNA and the polypeptide chain are co-linear.

Some proteins act as enzymes to catalyze biochemical reactions including the generation of energy and the synthesis of nucleotides and their assembly into nucleic acids. Other proteins are structural, or transport nutrients or take part in cell movement (mechanical proteins). Finally, there are proteins involved in information processing. Molecules whose primary role is to carry information (nucleic acids like DNA and messenger RNA) are basically linear molecules with a regular repeating structure. Molecules that form cellular structures or have active roles carrying out reactions are normally folded into three-dimensional (3-D) structures. These include both proteins and most non-translated RNA molecules, including tRNA and rRNA.

Proteins are made from a linear chain of monomers, known as amino acids (Fig. 3.24), and are folded into a variety of complex 3-D shapes. A chain of amino acids is called a polypeptide chain (Fig. 3.25).There are 20 different amino acids used in making proteins. All have a central carbon atom, the alpha carbon, surrounded by a hydrogen atom, an amino group (NH2), a carboxyl group (COOH), and a variable side chain, the R-group (Fig. 3.24). Amino acids are joined together by peptide bonds (Fig. 3.25). The first amino acid in the chain retains its free amino group and this end is often called the alpha carbon The central carbon atom of an amino acid, to which the amino, carboxyl and R groups are attached enzyme A protein or RNA molecule that catalyses a chemical reaction peptide bond Type of chemical linkage holding amino acids together in a protein molecule polypeptide chain Polymeric chain of amino acids

R-group Chemical group forming side chain of amino acid

The Structure of Proteins Has Four Levels of Organization 71

HO HO

Two amino acids eliminate „_, T_j,_J|_„„ „_, T_j,_JJ_„„

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