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Cover Biochemistry and Molecular Biology

Nitrogen metabolism: amino acid metabolism  

This chapter refers to amino acids which are supplied in the diet from protein hydrolysis in the gut. This includes proteins in the body that are constantly degraded and resynthesized. The chapter clarifies how the body can synthesize about ten of the amino acids. The rest must be obtained from the diet, but all 20 are needed for protein synthesis. Amino acids are also used to synthesize a wide variety of other molecules. The chapter discusses amino acids in excess of immediate requirements and shows that these are deaminated as the amino nitrogen is mainly converted into urea in mammals and excreted. The carbon-hydrogen skeletons are oxidized to release energy or converted into fat or glycogen according to the metabolic controls operating at the time and the particular amino acid.

Chapter

Cover Molecular Biology

Biological molecules  

This chapter reviews how molecules are built up by linking atoms together with covalent bonds and explores the way in which molecules interact with one another non-covalently in the aqueous environment of the cell. There are four major classes of biological molecules that play essential roles in all organisms: nucleotides, amino acids, carbohydrates, and lipids. Each of them can be found in cells both as individual small molecules or covalently linked to form larger molecules known as polymers or macromolecules. Nucleic acids are polymers of nucleotides that are responsible for carrying genetic information. Proteins, on the other hand, are polymers of amino acids that function as workhorses, carrying out most of the chemical reactions in the cell and giving cells their structure and shape. Many biological molecules can be covalently modified in ways that alter their chemical properties and allow their function to be regulated.

Chapter

Cover Biochemistry and Molecular Biology

Food digestion, absorption, and distribution to the tissues  

This chapter looks at digestion, wherein the polysaccharides and proteins of food are hydrolysed into their monomer subunits or simple sugars and amino acids in order to be absorbed by the intestinal epithelial cells in the bloodstream. The chapter looks at triacylglycerols or fats (TAGs) which are hydrolysed into fatty acids and monoacylglycerol, the process which is aided by bile salts that emulsify the fats to give a large surface area for the enzyme lipase to attack. The storage of fat is primarily in the adipocytes or fat cells of adipose tissue, where it occurs in large amounts as TAG. The chapter describes insulin, which is released in response to high glucose levels and stimulates fat and glycogen storage. Glucagon, released from the pancreas when blood glucose is low, stimulates release of glucose from the liver and of fatty acids from fat cells.

Chapter

Cover Biochemistry and Molecular Biology

Protein synthesis and controlled protein breakdown  

This chapter defines protein synthesis as the joining together of amino acids in the correct sequence to form a polypeptide chain. This involves three phases: initiation, chain elongation, and termination. The chapter analyzes messenger RNA (mRNA) and shows that it encodes the amino acid sequence of the protein in the form of triplets of bases known as codons. Of the 64 codons, AUG is the initiator or start codon, encoding methionine, and three codons are stop codons and the genetic code is degenerate as most amino acids are encoded by more than one codon. The chapter outlines the sequence of codons in an mRNA molecule. This is translated by cytosolic ribosomes. Ribosomes have a large and a small subunit, each containing RNA and many proteins.

Chapter

Cover Biochemistry and Molecular Biology

The structure of proteins  

This chapter focuses on proteins, which are made up of one or more polypeptide chains constructed from 20 species of amino acids. The length and sequence of each polypeptide is specified by its gene. The chapter describes 20 different amino acids which are of differing sizes and degrees of hydrophobicity, hydrophilicity, and electrical charge. These present the possibility of a vast variety of different proteins. The primary structure is the linear amino acid sequence, while the secondary structure involves folding of the polypeptide backbone. The chapter explains that the main secondary structure motifs are the α helix and the β-pleated sheet, which are stabilized by hydrogen bonding. The tertiary structure involves the further folding of the secondary structure motifs into the three-dimensional form of the protein.

Chapter

Cover Biochemistry and Molecular Biology

General principles of nutrition  

This chapter shows that nutrition is the science of food and the substances contained in it. The chapter notes that the main components of diet are macronutrients, which provide the bulk of the diet, and micronutrients, which include essential vitamins and minerals. The chapter explains that protein is needed to supply essential amino acids and fat is needed to supply essential fatty acids and provide sufficient energy in the diet. Most diets provide 10-15% of the energy in the form of protein and the remainder is made up of carbohydrate and fat. The chapter talks about the consumption of saturated fat, which is one of the risk factors for cardiovascular disease and some cancers, whereas consumption of polyunsaturated fat is considered healthy. A high consumption of sucrose can lead to dental caries.

Chapter

Cover Molecular Biology

Regulation of translation  

This chapter illustrates the regulation of translation. Translation initiation can be globally affected through the modification of core translational factors in eukaryotes. Both bacterial and eukaryotic organisms respond to amino acid limitation by shutting down overall protein synthesis, although the mechanisms differ. Initiation of specific bacterial mRNAs can be regulated through obstruction of the Shine–Dalgarno sequence by intrinsic RNA structure, metabolites, small RNAs, or proteins. Eukaryotic initiation is most often regulated through interactions of various factors (proteins and RNAs) with the 3' UTR (untranslated region) of the transcript. Meanwhile, translation elongation can be regulated either globally or gene-specifically. Viral systems have evolved ingenious mechanisms for slowing down host protein synthesis so that their own proteins can be more efficiently synthesized.