Macromolecules: Proteins, Nucleic Acids & ATP (Chapter 2, Sections 2.8–2.10)

Macromolecules: Proteins, Nucleic Acids & ATP

Lecture Objectives

This section introduces the major biological macromolecules—proteins, nucleic acids, and ATP—focusing on their structure, function, and importance in human physiology.

• Proteins: Examples and functions in the human body

• Enzymes: Role in metabolic reactions

• Macromolecule Synthesis & Breakdown: Chemical reactions involved

• Nucleic Acids: Structure and function of DNA and RNA

• ATP: Structure and biological use

Proteins

Amino Acids: The Building Blocks

Proteins are long chains of single units called amino acids. There are 20 different amino acids used in humans.

• Each amino acid contains an amino group (-NH3) at one end and a carboxyl group (-COOH) at the other.

• The human body can synthesize 11 of these amino acids; the remaining 9 are essential amino acids and must be obtained from the diet.

Polypeptides and Proteins

A polypeptide is a chain of 3–100 amino acids joined by dehydration synthesis. Chains longer than 100 amino acids are generally called proteins, which have complex structures and functions.

• Dehydration synthesis is the process by which amino acids are linked together, releasing water as a byproduct.

Figure 1: Dehydration Synthesis of Amino Acids

Dehydration synthesis reactions occur between each amino acid to bond them together into polypeptides.

Equation:

$ext{Amino Acid}_1 + ext{Amino Acid}_2 \rightarrow ext{Dipeptide} + H_2O$

Functions of Proteins in Humans

Proteins serve thousands of unique functions in the human body:

• Structural support: e.g., collagen in connective tissues

• Muscle contraction: e.g., actin and myosin in muscle fibers

• Cell membrane components: e.g., membrane proteins for transport and signaling

• Transmission of information: e.g., receptor proteins

• Transport of materials: e.g., hemoglobin transporting oxygen

• Enzymes: regulation of biochemical reactions

Enzymes

Definition and Role

Enzymes are proteins that function as biological catalysts, speeding up chemical reactions without being consumed or altered in the process.

• Without enzymes, many life-sustaining reactions would occur too slowly to support life.

• Enzymes are highly specific for their substrates and reactions.

Example: Digestion

• Humans possess enzymes to digest glycogen and starch, but lack enzymes to digest cellulose.

Chemical Reactions in Macromolecule Synthesis & Breakdown

Dehydration Synthesis and Hydrolysis

Macromolecules are formed and broken down by two key types of chemical reactions:

• Dehydration synthesis: Joins monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

Equation for Hydrolysis:

$ext{Polypeptide} + H_2O \rightarrow ext{Amino Acid}_1 + ext{Amino Acid}_2$

Nucleic Acids

Types and Functions

Nucleic acids are molecules that store and transmit genetic information. The two main types are:

• Deoxyribonucleic acid (DNA): Contains deoxyribose sugar

• Ribonucleic acid (RNA): Contains ribose sugar

DNA contains instructions for producing RNA, which in turn contains instructions for producing proteins. Proteins direct most of life’s processes.

Nucleotides: The Subunits

Both DNA and RNA are composed of smaller units called nucleotides. Each nucleotide consists of:

• A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• One or more phosphate groups

• A nitrogen-containing base (single or double ringed)

There are eight different nucleotides, distinguished by their bases:

• DNA bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

• RNA bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

Table: DNA vs. RNA Nucleotides

Type	Sugar	Bases	Strandedness
DNA	Deoxyribose	A, T, C, G	Double-stranded
RNA	Ribose	A, U, C, G	Single-stranded

Structure of DNA

• Double helix formed by two intertwined strands

• Paired bases: Adenine pairs with Thymine, Cytosine pairs with Guanine

• Strands held together by hydrogen bonds between bases

• Nucleotides linked by covalent bonds between phosphate and sugar groups

Structure of RNA

• Single-stranded molecule

• Uracil replaces thymine as a base

• Acts as a complementary copy of a segment of DNA, used to produce proteins

• Shorter in length than DNA

Adenosine Triphosphate (ATP)

Structure and Function

ATP is a modified nucleotide that serves as the universal energy source for cells.

• ATP is identical to an adenine-containing nucleotide in RNA, except it has two additional phosphate groups (three total).

• High-energy bonds between phosphate groups store potential energy.

• Energy is released for cellular use when the bond between the last two phosphates is broken via hydrolysis.

Equation for ATP Hydrolysis:

$\text{ATP} + H_2O \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

Where ADP is adenosine diphosphate and Pi is inorganic phosphate.

Biological Importance

• ATP powers cellular processes such as muscle contraction, active transport, and biosynthesis.

• ATP is continuously regenerated from ADP and phosphate in cells.

References: Johnson, M.D. (2017). Human Biology: Concepts and Current Issues (8th ed.). Pearson Education Inc.